Optical assemblies for free-space optical propagation between waveguide(s) and/or fiber(s)

ABSTRACT

An optical apparatus comprises a substrate, first and second transmission optical elements on the substrate, and an optical component (such as an isolator) and focusing optical element(s) on the substrate between the transmission elements. Transmission elements may include planar waveguide(s) formed on the substrate and/or optical fiber(s) mounted in groove(s) on the substrate. The focusing element(s) may include: gradient-index (GRIN) segment(s) mounted on the substrate or spliced onto a fiber, a focusing segment(s) of a planar waveguide, ball lens(es), aspheric lens(es), and/or Fresnel lens(es). A dual-lens optical assembly comprises a pair of GRIN segments secured to a substrate in one or more grooves, and may be formed from a common length of GRIN optical medium. An optical component (such as an isolator) is positioned between the paired GRIN segments, and optical power is transmitted by the dual-lens assembly between planar waveguide(s) and/or fiber(s) through the optical component.

RELATED APPLICATIONS

[0001] This application claims benefit of the following U.S. provisionalpatent applications:

[0002] App. No. 60/413,986 entitled “Free-space optical propagationbetween a waveguide and a fiber” filed Sep. 25, 2002 in the names ofHenry A. Blauvelt, David W. Vernooy, and Joel S. Paslaski, saidprovisional application being hereby incorporated by reference as iffully set forth herein;

[0003] App. No. 60/455,712 entitled “Optical assemblies for free-spaceoptical propagation between waveguide(s) and/or fiber(s)” filed Mar. 17,2003 in the names of Henry A. Blauvelt and David W. Vernooy, saidprovisional application being hereby incorporated by reference as iffully set forth herein; and

[0004] App. No. 60/466,799 entitled “Low-profile-core and thin-coreoptical waveguides and methods of fabrication and use thereof” filedApr. 29, 2003 in the names of David W. Vernooy, Joel S. Paslaski, andGuido Hunziker, said provisional application being hereby incorporatedby reference as if fully set forth herein.

BACKGROUND

[0005] The field of the present invention relates to free-space opticalpower transfer. In particular, optical assemblies for free-space opticalpropagation between waveguide(s) and/or fiber(s), and fabricationmethods therefor, are disclosed herein.

[0006] Many optical components cannot yet be implemented within awaveguide or optical fiber, but require so-called “free-space”propagation of optical power through the component. The transversedimensions of such components are typically too large to providetransverse confinement or guiding of the propagating optical power,which will converge and/or diverge as it propagates through thecomponent. When such components must be incorporated into an opticaltransmission system that includes one or more planar-waveguide(s) and/oroptical fiber(s), additional focusing and/or collection optics arerequired for: 1) transforming a small guided mode emerging from an endof an optical fiber or planar waveguide (typically less than about 10 μmacross and divergent upon leaving the fiber or waveguide) into afree-space optical mode that may be transmitted through the opticalcomponent; and/or 2) collecting the free-space optical mode andtransforming it into an optical mode (typically convergent) that may beefficiently coupled into another optical fiber or planar waveguide. Theoverall efficiency of optical power transfer between thefiber(s)/waveguide(s) is determined to a major extent by the degree ofspatial mode matching achieved between the fiber/waveguide optical modesby the additional focusing and/or collection optics.

[0007] Exemplary prior art dual-lens optical assemblies are shown inFIGS. 1 and 2, where an optical isolator 40 (comprising in this examplea Faraday rotator with input and output polarizers cemented onto thefaces thereof) is shown positioned between two lenses 22 and 72 (balllenses in FIG. 1, spaced from the fiber ends as shown or alternativelyin contact with the fiber ends; gradient-index [GRIN] optical fibercoupling segments fusion spliced onto the fiber ends in FIG. 2). Theoptical modes are approximately indicated by the dashed lines in FIGS. 1and 2. Optical power propagating through a single-mode optical fiber 20exits the fiber end and is then focused by lens 22 for propagationthrough isolator 40 (with decreased divergence, substantiallycollimated, or convergent). Once through the isolator 40, thepropagating optical power (typically, but not necessarily, divergent atthis point) is collected and coupled into single-mode fiber 70 by lens72. Optical transmission between fiber 20 and fiber 70 through isolator40 is kept above operationally acceptable levels (i.e., the lensesprovide adequate spatial mode matching between the two fibers) onlywithin tight longitudinal, transverse, and angular alignment tolerancesfor both fiber ends and lenses (typically a few μm or less). Achievingalignment within these tolerances typically requires expensive andtime-consuming active alignment procedures, driving up costs forassembled devices (“active alignment” denoting a procedure in whichoptical power transmission through the fibers/lenses is monitored forguiding the alignment procedure; in contrast, a “passive alignment”procedure does not require optical power transmission during thealignment procedure). Furthermore, while the solutions shown in FIGS. 1and 2 may be adequate for some in-line fiber-optic applications, thereis also a need for solutions compatible with semiconductor-based activeoptical devices, such as lasers and modulators, and/or compatible withplanar waveguide optical transmission components. Optical mode sizes inthese cases may be smaller (sometimes less than 1-2 μm across) anddivergences correspondingly larger, imposing even tighter alignmenttolerances for achieving an operationally acceptable level of opticalpower transfer.

[0008] Various exemplary embodiments of single- and dual-lens opticalassemblies and methods for constructing the same are disclosed hereinwhich may overcome one or more of the drawbacks of the previous art (asdescribed hereinabove).

SUMMARY

[0009] An optical apparatus comprises a substrate, first and secondtransmission optical elements positioned on the substrate, a“free-space” optical component mounted on the substrate between theproximal ends of the transmission optical elements, and at least onefocusing optical element mounted on the substrate between the proximalends of the transmission optical elements for transmitting optical powerbetween them through the optical component. The transmission opticalelements may comprise planar waveguide(s) formed on the substrate and/oroptical fiber(s) mounted in groove(s) on the substrate. The focusingelement(s) may comprise one or more of: gradient-index (GRIN) segment(s)mounted on the substrate or spliced onto a fiber, a focusing segment ofa planar waveguide, ball lens(es), aspheric lens(es), and/or Fresnellens(es). One or more optical paths between transmission opticalelements, optical component, and/or focusing optical element(s) may befilled with a transparent embedding medium, which may also serve toencapsulate or hermetically seal the optical apparatus. An opticalisolator is an example of an optical component that may be incorporatedinto the optical apparatus, and the apparatus may form a portion of anoptical assembly or sub-assembly such as a transmitter, receiver,transceiver, laser, and so forth.

[0010] A dual-lens optical apparatus comprises a pair of GRIN segmentssecured to a substrate in one or more grooves. The GRIN segments aresubstantially parallel and longitudinally spaced apart on the substrate,and may be formed from a common length of GRIN optical medium secured tothe substrate and then divided in to the GRIN segments. A “free-space”optical component is positioned between the paired GRIN segments. Thedual-lens optical apparatus is employed for transmitting optical powerbetween first and second transmission optical elements (e.g., planarwaveguides and/or fiber(s)) through the optical component. Opticalfibers may be secured in grooves on the substrate, or the apparatus maybe mounted on a second substrate with the GRIN fiber segments ingroove(s) thereon. Planar waveguide(s) and/or optical fiber(s) may alsobe positioned on the second substrate. An embedding medium may fill oneor more optical paths between the optical component, GRIN segments,waveguide(s), and/or fiber(s), and may also serve to encapsulate orhermetically seal the dual-lens optical apparatus. An optical isolatoris an example of an optical component that may be incorporated into thedual-lens optical apparatus, and the dual-lens apparatus may form aportion of an optical assembly or sub-assembly such as a transmitter,receiver, transceiver, laser, and so forth.

[0011] Objects and advantages pertaining to free-space opticalpropagation between waveguide(s) and/or fiber(s) may become apparentupon referring to the disclosed exemplary embodiments as illustrated inthe drawings and set forth in the following written description and/orclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 illustrates a prior arrangement for free-space propagationof optical power through an optical component between optical fibers.

[0013]FIG. 2 illustrates a prior arrangement for free-space propagationof optical power through an optical component between optical fibers.

[0014]FIGS. 3A and 3B are side and top views, respectively, of anexemplary fabrication/assembly sequence for a dual-lens opticalassembly. FIG. 3C is a top view of the same exemplary sequenceimplemented on a wafer scale.

[0015]FIGS. 3D and 3E are side and top views, respectively, of anexemplary fabrication/assembly sequence for a dual-lens opticalassembly. FIG. 3F is a top view of the same exemplary sequenceimplemented on a wafer scale.

[0016]FIGS. 4A and 4B are side and top views, respectively, of anexemplary optical assembly and optical component. FIG. 4C is a side viewof an exemplary optical assembly and optical component.

[0017]FIGS. 5A and 5B are side and top views, respectively, of anexemplary optical assembly, optical component, and optical fibers. FIG.5C is a side view of an exemplary optical assembly, optical component,and optical fibers.

[0018]FIGS. 6A and 6B are side and top views, respectively, of anexemplary optical assembly, optical component, and planar waveguides.FIG. 6C is a side view of an exemplary optical assembly, opticalcomponent, and planar waveguide.

[0019]FIGS. 7A and 7B are side and top views, respectively, of anexemplary optical assembly, optical component, optical fiber, and planarwaveguide. FIG. 7C is a side view of an exemplary optical assembly,optical component, optical fiber, and planar waveguide.

[0020]FIGS. 8A and 8B are side and top views, respectively, of anexemplary fabrication/assembly sequence for a dual-lens opticalassembly.

[0021]FIGS. 9A and 9B are side and top views, respectively, of anexemplary fabrication/assembly sequence for a dual-lens opticalassembly.

[0022]FIG. 10 is a top view of an exemplary fabrication/assemblysequence for a dual-lens optical assembly.

[0023]FIG. 11 is a top view of an exemplary optical assembly, opticalcomponent, and optical fibers.

[0024]FIG. 12 is a top view of an exemplary optical assembly, opticalcomponent, and planar waveguides.

[0025]FIG. 13 is a top view of an exemplary optical assembly, opticalcomponent, optical fiber, and planar waveguide.

[0026]FIG. 14 is a side view of an exemplary optical assembly, opticalcomponent, and planar waveguides.

[0027]FIG. 15 is a side view of an exemplary optical assembly, opticalcomponent, optical fiber, and planar waveguide.

[0028]FIG. 16 is a top view of an exemplary fabrication/assemblysequence for a GRIN lens optical assembly.

[0029]FIG. 17 is a side view of exemplary GRIN lenses, opticalcomponent, and planar waveguides.

[0030]FIG. 18 is a side view of exemplary GRIN lenses, opticalcomponent, optical fiber, and planar waveguide.

[0031]FIG. 19 is an end cross-sectional view of an exemplaryfabrication/assembly sequence for a dual-lens optical assembly.

[0032]FIGS. 20A and 20B are top and side views, respectively, of anexemplary assembly of an optical component, optical fiber, and planarwaveguide.

[0033]FIGS. 21A and 21B are top views of exemplary assemblies of aplanar waveguide and optical fiber, with and without an opticalcomponent, respectively.

[0034]FIGS. 22A and 22B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, andoptical fiber.

[0035]FIGS. 23A and 23B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, andoptical fibers.

[0036]FIGS. 24A and 24B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, opticalfiber, and lenses.

[0037]FIGS. 25A and 25B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, opticalfiber, and lens.

[0038]FIGS. 26A and 26B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, opticalfiber, and lens.

[0039]FIG. 27 is a side view of an exemplary GRIN lens, opticalcomponent, and planar waveguides.

[0040]FIG. 28 is a side view of an exemplary GRIN lens, opticalcomponent, optical fiber, and planar waveguide.

[0041]FIG. 29 is a side view of an exemplary GRIN lens, opticalcomponent, optical fiber, and planar waveguide.

[0042]FIGS. 30A and 30B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, opticalfiber, and lenses.

[0043]FIGS. 31A and 31B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, opticalfiber, and lens.

[0044]FIGS. 32A and 32B are top and side views, respectively, of anexemplary assembly of an optical component, planar waveguide, opticalfiber, and lens.

[0045]FIG. 33 is a schematic diagram of a laser source with an opticalassembly.

[0046]FIG. 34 is a schematic diagram of an optical transceiver with anoptical assembly.

[0047]FIG. 35 is a schematic diagram of a photodetector with an opticalassembly.

[0048] It should be noted that the relative proportions of variousstructures shown in the Figures may be distorted to more clearlyillustrate the present invention. Relative dimensions of various opticaldevices, optical waveguides, optical fibers, optical components, opticalmodes, alignment/support members, grooves, and so forth may bedistorted, both relative to each other as well as in their relativetransverse and/or longitudinal proportions. In many of the Figures thetransverse or longitudinal dimension of one or more elements isexaggerated relative to the other dimension for clarity.

[0049] The embodiments shown in the Figures are exemplary, and shouldnot be construed as limiting the scope of the present disclosure and/orappended claims.

DETAILED DESCRIPTION OF EMBODIMENTS

[0050] Exemplary fabrication/assembly sequences are illustrated in FIGS.3A-3F, each for producing an exemplary dual-lens optical assembly 200.In each sequence, a substrate 250 is provided with an elongated groove252, typically a V-groove, using spatially selective materialprocessing. For simultaneous fabrication of multiple assemblies on awafer scale, multiple substantially parallel grooves 252 may be providedon substrate 250 (FIGS. 3C and 3F). In a first exemplary sequence (FIGS.3A-3C), recessed areas 251 and 253 a/253 b are formed on substrate 250using spatially selective material processing. These recessed areasdivide the V-groove(s) 252. Multiple recessed areas 251/253 a/253 b maybe formed dividing multiple V-grooves 252 each into multiple segments asin FIG. 3C, for a fabrication/assembly sequence implemented on a waferscale. Separate recessed areas may be formed (not shown), or groups ofrecessed areas may be formed together as slots or grooves running acrossthe substrate 250 substantially perpendicular to V-grooves 252 (as inFIG. 3C). In either case, if needed or desired the recessed areas 251may be further adapted for later positioning and/or alignment of anoptical component (by providing alignment structures in and/or near therecessed area, for example). Similarly, recessed areas 253 a/253 b maybe further adapted, if needed or desired, for later positioning and/oralignment of the dual-lens optical assembly relative to waveguide(s)and/or fiber(s). A length of gradient-index (GRIN) multi-mode opticalfiber 220 is positioned within each V-groove 252 and secured tosubstrate 250. The GRIN optical fiber 220 thus positioned and securedspans the recessed area(s) 251/253 a/253 b. The GRIN fiber 220 iscleaved to remove portions spanning recessed areas 251/253 a/253 b,thereby forming GRIN fiber segments 220 a and 220 b. The position ofeach fiber cleave may substantially coincide with a corresponding edgeof a recessed area 251/253 a/253 b, may leave a slight overhang of thefiber segment over the edge of the recessed area (on the order of a fewμm, corresponding roughly to the position accuracy/tolerance of thecleaving process), or may leave a substantial length of GRIN fibersegment overhanging the edge of the recessed area. Each GRIN fiber maybe cleaved to form multiple pairs of GRIN fiber segments 220 a/220 b toin turn form multiple dual-lens assemblies along the length of thegroove(s) 252 (FIG. 3C). In some instances a recessed area 253 b for oneoptical assembly may also serve as recessed area 253 a for an adjacentassembly along the groove 252; in other cases each assembly is providedwith its own pair of recessed areas 253 a/253 b. The GRIN optical fibersegments 220 a and 220 b serve as the lenses of the dual-lens opticalassembly 200. The separation between segments 220 a and 220 b and thelengths of the segments 220 a and 220 b are substantially determined bythe position accuracy of the cleaving process.

[0051] A second exemplary fabrication/assembly sequence for producing anexemplary dual-lens optical assembly 200 is illustrated in FIGS. 3D-3F.Using spatially selective material processing, a substrate 250 isprovided with an elongated groove 252, typically a V-groove. If neededor desired, substrate 250 may be further provided with recessed areas255 adapted for later positioning and/or alignment of an opticalcomponent (including alignment structures in and/or near the recessedarea 255, if needed or desired). For simultaneous fabrication ofmultiple assemblies on a wafer scale, multiple substantially parallelV-grooves 252 may be provided on substrate 250 (FIG. 3F). A length ofgradient-index (GRIN) multi-mode optical fiber 220 is positioned withineach V-groove 252 and secured to substrate 250. A set of substantiallyparallel precision saw cuts 254, 256 a, ad 256 b are made substantiallyperpendicular to V-grooves 252. Multiple sets of saw cuts 254/256 a/256b may be made to form multiple dual-lens assemblies along the length ofthe V-groove(s) 252 (FIG. 3F; recessed areas 255 omitted for clarity).In some instances a saw cut 256 b for one optical assembly may alsoserve as saw cut 256 a for an adjacent assembly along the groove 252; inother cases each assembly is provided with its own pair of saw cuts 256a/256 b. Saw cuts 254 and 256 a/256 b are typically sufficiently deep soas to completely sever optical fiber 220, and are often at least aboutas deep as V-groove 252, or deeper if needed or desired. Saw cuts 254are typically not as deep as recessed areas 255 (if present). Saw cuts254 are sufficiently wide so as to accommodate within a later-placedoptical component (not shown in FIGS. 3A-3C). If recessed areas 255 arenot present, saw cuts 254 may serve to receive, position, and/or align alater-placed optical component. Saw cuts 254 do not extend through theentire thickness of substrate 250.

[0052] The saw cuts 254 and 256 a/256 b divide the GRIN optical fiberinto segments 220 a and 220 b, which serve as the lenses of thedual-lens optical assembly 200. Any remaining GRIN fiber segments(between saw cut 256 b of one optical assembly and saw cut 256 a of anadjacent assembly) may be removed or discarded, if needed or desired.The separation between segments 220 a and 220 b is substantiallydetermined by the width of saw cuts 254, while the lengths of thesegments 220 a and 220 b are substantially determined by the positionsof saw cuts 256 a/256 b and the width of saw cuts 254. It should benoted that saw cuts 254 and/or 256 a/256 b may be formed by a singlepass of a saw with the width of the cut determined by the width of theblade employed. Alternatively, saw cuts 254 and/or 256 a/256 b may beformed by multiple saw passes, using one or more blades, and the widthof the final saw cut determined by the positions of the outer edges ofthe blades used for the outermost passes (which may be deeper than therest, to facilitate later positioning of other components within groove254 against a side edge thereof). Alternatively, saw cuts 254 and/or 256a/256 b may be formed by two thinner substantially parallel saw cutsthat form the side edges of the main cut, with the substrate materialremaining therebetween removed by some other suitable spatiallyselective material processing technique.

[0053] If no further processing is required, the substrate wafer may bedivided into individual substrate chips 250 (regardless of which of theforegoing sequences is employed, or if another functionally equivalentsequence is employed). Each individual substrate chip has thereon adual-lens optical assembly 200 comprising a pair of GRIN fiber segments220 a and 220 b (positioned within respective groove segments 252 a and252 b) separated by a portion of a saw cut 254 or by a portion of arecessed area 251. If saw cuts are employed, saw cuts 256 a/256 b mayextend through substrate 250 to divide the substrate wafer into stripsor bars, which may be subsequently divided into individual substratechips (if the substrate wafer originally had multiple grooves 252). Ifsaw cuts 256 a/256 b do not extend through substrate 250, or if saw cutsare not employed, then the substrate wafer must be divided into bars orstrips, and the bars then divided into individual substrate chips by anysuitable method (once again, assuming multiple grooves 252 with multiplefibers 220). Portions of recessed areas 253 a/253 b, or portions of sawcuts 256 a/256 b, may remain on the individual substrate chip 250.

[0054] If further processing of the optical assembly 200 is required,such processing may be performed after dividing the substrate wafer intostrips (i.e., at the bar level), or after dividing the bars intoindividual substrate chips (i.e., at the chip level). It may beadvantageous, however, to perform further processing on a wafer scale,before any division of the substrate wafer occurs, for multipledual-lens optical assemblies simultaneously. Such additional processingsteps for the dual-lens optical assemblies may include one or more of,but are not limited to: altering the end-face surface profile; improvingend-face surface optical quality; applying one or more optical coatings;and so forth.

[0055] The exemplary procedures illustrated in FIGS. 3A-3F yield adual-lens optical assembly 200 (many dual-lens optical assemblies 200 ifimplemented on a wafer scale) comprising GRIN fiber segments 220 a and220 b positioned in respective groove segments 252 a and 252 b on chipsubstrate 250. Since GRIN optical fiber segments 220 a and 220 b comefrom a common optical fiber 220 positioned in a common groove 252, thelenses formed by fiber segments 220 a and 220 b are positionedsubstantially coaxially. Any axial asymmetry (i.e., off-center GRINprofile) of GRIN fiber 220 is self-correlated, since the GRIN fibersegments 220 a and 220 b are formed from nearly adjacent segments of thesame fiber and are positioned within their respective groove segments252 a and 252 b before GRIN fiber 220 is separated into segments 220 aand 220 b. Potential transverse misalignment of the lenses of thedual-lens assembly is thereby reduced or substantially eliminated. Theseparation of the lenses is determined by the positions of fiber cleaves(FIGS. 3A-3C) or by the width of saw cut 254 (FIGS. 3D-3F), either ofwhich may be pre-determined within about 10 micron accuracy. Potentiallongitudinal misalignment of the lenses (relative to one another) isthereby reduced or substantially eliminated.

[0056] The focal properties of the lenses are determined by thegradient-index transverse profile of the GRIN optical fiber 220, and thelengths of the fiber segments 220 a and 220 b. The index profiles forvarious commercially available GRIN multimode optical fibers are wellcharacterized. For a given GRIN fiber (with a given GRIN transverseprofile), the correct length for the fiber segment may be calculatedbased on the particular spatial mode matching problem being addressed bythe dual-lens optical assembly (using standard optical designtechniques), and may typically range anywhere between about 100 μm toseveral mm in length. The mode sizes and diffractive properties of thewaveguide(s) and/or fiber(s) to be coupled, as well as the operatingwavelength, the index and other optical characteristics of the opticalcomponent, and the index of the surrounding medium, are all used tocalculate the focal properties for the GRIN fiber segments 220 a and 220b that are required to achieve an operationally acceptable level ofoptical power transfer (i.e., a sufficient degree of spatial madematching) from one waveguide/fiber, through the dual-lens opticalassembly and the optical component, and into the other waveguide/fiber.The length tolerances required for achieving the desired focalproperties for the GRIN lens segments is typically on the order of aboutten microns, consistent with the precision achievable for positioningfiber cleaves (FIGS. 3A-3C) or saw cuts 254 and 256 a/256 b (FIG.3D-3F).

[0057] In FIGS. 4A and 4B, a so-called free-space optical component 300is positioned between the GRIN fiber segments 220 a and 220 b inrecessed area 251, positioned and/or aligned by any alignment structuresprovided in or near the recessed area 251 (alignment structures notshown). The following description/discussion may equivalently apply toan optical component 300 placed within a saw cut 254 or a recessed area255 therein. The first and second end faces of optical component 300each face the proximal end face of the first and second GRIN fibersegments 220 a and 220 b, respectively. An exemplary optical component300 may be an optical isolator, typically including a Faraday rotatorcrystal configured for non-reciprocal 45 ° rotation placed between apair of linear polarizers with their transmission axes offset by 45°.The polarizers may be Polarcor® or other bulk polarizers cemented orotherwise secured to the faces of the Faraday rotator, or may beprovided as thin film coatings on the faces of the Faraday rotator.Other free-space optical components may be placed between the lensesinstead. The dual-lens optical assembly 200 and/or the optical component300 may be adapted for positioning and securing the optical component300 between the lens segments 220 a and 220 b. Due to the “free-space”nature of the optical component 300, transverse alignment of the opticalcomponent relative to the lens segments is typically non-critical. It istypically sufficient to make the transverse dimensions of the opticalcomponent sufficiently large so that positioning the component on thebottom of the recessed area 251 (or saw cut 254 or recessed area 255thereof, as the case may be) and substantially centered (within thetolerance of the positioning apparatus employed) results insubstantially unimpeded transmission between the lens segments throughthe optical component. Optical component(s) 300 may be positioned andsecured between GRIN fiber segments 220 a and 220 b before division ofthe substrate wafer (wafer scale, realizing significant economies ofmanufacture), after cutting the substrate wafer into strips (bar level),or after division into individual assembly chips (chip level).

[0058] The refractive index of optical component 300 must besubstantially well characterized, and serves as an input for designingthe focal properties of the lens segments 220 a and 220 b. Thethickness, longitudinal positioning, and angular alignment of opticalcomponent 300 are related to one another and to the width of recessedarea 251 (or saw cut 254 or recessed area 255 thereof, as the case maybe). The overall thickness of optical component 300 determines theminimum separation of lens segments 220 a and 220 b, and serves asanother input for calculating the desired focal properties of the lenssegments. Recessed area 251 may be wider than the thickness of opticalcomponent 300, and substrate 250, recessed area 251, and/or opticalcomponent 300 may be adapted for enabling sufficiently accurate angularalignment and longitudinal positioning (FIGS. 4A-4C;positioning/alignment adaptations not shown). Suitable adaptations mayinclude, but are not limited to: alignment edges and/or otherregistering surfaces; interlocking surfaces; alignment marks or targets;kinematic alignment structures (such as grooves, recesses, protrusions,and the like); and so on. Alternatively, optical component 300 may bepositioned within the recessed area 251 against one side edge thereof,thereby reducing or substantially eliminating angular misalignment andvariations of longitudinal position. Alternatively, recessed area 251may be may only just wide enough (within fabrication tolerances) toaccommodate optical component 300 between GRIN fiber segments 220 a and220 b, thereby reducing or substantially eliminating angularmisalignment and variations in longitudinal position. Once properlypositioned, optical component 300 may be secured within recessed area251 between GRIN lens segments 220 a and 220 b by any suitable means,including but not limited to: adhesives; embedding media (includingencapsulants, polymers, and so forth); one or more clamps, retainers,covers or lids, or other structural components; solder; electrostaticinteractions; heat/pressure bonding; and so forth.

[0059] It may be advantageous to provide one or both surfaces of theoptical component 300 with anti-reflection coatings and/or other opticalcoatings, similar to coating mentioned hereinabove that might beprovided on the transmissive surfaces (i.e., end faces) of the GRIN lenssegments 220 a and 220 b. Alternatively, the optical path between theGRIN lens segments and adjacent surfaces of the optical component 300may be substantially filled with an index-matching embedding medium(such as a polymer, for example). Such a medium may typically be chosento have an index near or between the indices of the GRIN lens segmentand the optical component, but may be chosen with any refractive indexthat serves to reduce unwanted reflections at the surfaces relative tovacuum or ambient air. The index-matching medium may be selectivelyapplied to embed the relevant optical surfaces (i.e., end faces), orinstead may be employed as an encapsulant 454 for the entire opticalcomponent 300 and the adjacent portions of GRIN lenses 220 a and 220 b(as in FIG. 4C; encapsulant may also serve as a hermetic sealant). Whenused as an encapsulant, the index-matching embedding medium may alsofulfill the function of securing the optical component 300 in its properposition within recessed area 251, and may further serve as amechanical, moisture, chemical, and/or hermetic protective barrier.

[0060] However optical component 300 is incorporated into dual-lensoptical assembly 200, the result is a substantially internally-alignedoptical assembly. Such an optical assembly 200 and optical component 300may be designed as described hereinabove for a particular pair ofoptical transmission waveguide(s) and/or fiber(s). Such an assembly maybe positioned for receiving incident optical power from a first opticaltransmission waveguide/fiber in a first optical mode, transmitting thatoptical power through the optical component, and delivering thetransmitted optical power substantially spatial mode matched (withinoperationally acceptable limits) with an optical mode of a secondoptical transmission waveguide/fiber. Specific examples are describedhereinbelow, but are only examples, and shall not limit the scope of thepresent disclosure.

[0061]FIGS. 5A-5C illustrate an exemplary dual-lens optical assemblyadapted for transferring optical power between single mode opticalfibers 520 a and 520 b. Optical fibers 520 a and 520 b are typicallysubstantially identical, but this need not be the case. The GRIN fibersegments 220 a and 220 b are of a length suitable for substantially modematching (at an operationally acceptable level) an optical modetransmitted through the proximal end face of single mode fiber 520 a andan optical mode transmitted through the proximal end face of single modefiber 520 b (given the separation of the GRIN fiber segments 220 a and220 b, the thickness and index of the optical component 300, and thethickness and index of any medium between the GRIN fiber segments andthe optical component 300). Recessed areas 251/253 a/253 b are shown inFIGS. 5A-5C; saw cuts 254/256 a/256 b (and recessed area 255, ifpresent) could be equivalently employed. Excess GRIN fiber segmentsbetween adjacent recessed areas 253 a/253 b are removed, leaving emptysegments of groove(s) 252. The division of substrate wafer 250 iscarried out so that areas of substrate 250 with empty segments 552 a and552 b of groove 252 remain on either side of the pair of GRIN fibersegments 220 a and 220 b. Optical component(s) 300 may be positioned andsecured between GRIN fiber segments 220 a and 220 b before division ofthe substrate wafer (wafer scale), after cutting the substrate waferinto strips (bar level), or after division into individual assemblychips (chip level).

[0062] Single mode optical fibers 520 a and 520 b are positioned withinempty groove segments 552 a and 552 b, respectively, and pushed alongthe respective groove segment to the desired longitudinal position.Single mode fibers 520 a and 520 b may be secured within groove segments552 a and 552 b using: adhesives; embedding media (includingencapsulants, polymers, and so forth); one or more clamps, retainers,covers or lids, or other structural components; solder; electrostaticinteractions; heat/pressure bonding; and so forth. An index-matchingembedding medium (such as a polymer, for example) may be employedbetween the proximal end faces of single mode fibers 523 aand 520 b andthe distal end faces of respective GRIN fiber segments 220 a and 220 bto reduce or substantially eliminate unwanted reflections and increaseoverall optical throughput. Such an index-matching embedding medium mayhave an index near or between the indices of the GRIN fiber segments andthe single mode fibers, although any medium that reduces thereflectivity at the various optical surfaces could be employed. Theindex-matching embedding medium may be the same as that employed (ifany) between proximal end faces of the GRIN fiber segments and the endfaces of optical component 300, or may be a different index matchingmedium. The index-matching embedding medium may be selectively appliedto embed the relevant optical surfaces (i.e., end faces), or may beemployed as an encapsulant 554 for optical component 300, GRIN lenses220 a and 220 b, and the adjacent portions of single mode fibers 520 aand 520 b (as in FIG. 5C; encapsulant may also serve as a hermeticsealant). An index-matching embedding medium or encapsulant may alsoserve to secure the single mode fibers within the respective groovesegments and/or to secure/index-match component 300 between GRINsegments 220 a and 220 b (FIG. 5C).

[0063] This exemplary fabrication scheme enables sufficiently accurateand substantially reproducible/repeatable transverse alignment betweensingle mode optical fibers 520 a and 520 b and the respective GRIN fibersegments 220 a and 220 b. The optical modes are approximately indicatedby the dashed lines in FIGS. 5A and 5B. Optical fiber, both single modeand GRIN multimode, are extraordinarily well-characterized commercialproducts, and the outer diameters of the fibers may be quite accuratelyknown. By employing single mode fibers 520 a and 520 b and GRIN fiber220 having substantially equal outer diameters, and placing them in acommon substrate V-groove formed by spatially selective materialprocessing, the single mode fiber end and the GRIN fiber segments may besubstantially coaxial (both mechanically and optically), and thereforemay be transversely positioned sufficiently accurately for enabling anoperationally acceptable degree of optical power transfer. Forlongitudinal positioning of single mode fibers 520 a and 520 b,alignment stops and/or alignment marks (none shown) may be formed withinor near the groove segments 552 a and 552 b or within an adjacentportion of recessed portions 253 a/253 b. The proximal ends of singlemode fibers 520 a and 520 b may butt against corresponding alignmentstops and/or may be aligned with corresponding alignment marks forsufficiently accurate longitudinal positioning relative to therespective GRIN fiber segment without making contact therewith.Alternatively, the proximal end face of each of single mode fibers 520 aand 520 b may butt directly against the distal end face of thecorresponding GRIN fiber segment for longitudinal positioning.

[0064] It may be desirable to implement an embodiment analogous to thatshown in FIGS. 5A-5C (i.e., with the optical assembly between twooptical fibers), in which the dual-lens optical assembly 200 and theoptical fibers 520 a and 520 b are all mounted on a second substratewith a groove (embodiment not shown). Optical component 300 may bemounted on substrate 250, or on the second substrate. Securing the GRINoptical fiber segments 220 a and 220 b and the optical fibers 520 a and520 b in a common groove results in substantially coaxial alignment, asdescribed above. Such embodiments shall fall within the cope of thepresent disclosure and/or appended claims.

[0065]FIGS. 6A-6C illustrate an exemplary dual-lens optical assemblymounted on a planar waveguide substrate 721 between a first planaroptical waveguide 720 a and a second planar optical waveguide 720 b(substantially collinear with and longitudinally spaced-apart fromwaveguide 720 a). Planar optical waveguides 720 a and 720 b may be ofany suitable type and configured in any suitable way, described furtherhereinbelow. The dual-lens assembly is adapted for transferring opticalpower between the planar waveguides through optical component 300. TheGRIN fiber segments 220 a and 220 b are of lengths suitable forsubstantially mode matching (at an operationally acceptable level) anoptical mode transmitted through the proximal end face of planarwaveguide 720 a and an optical mode transmitted through the proximal endface of planar waveguide 720 b (given the separation of the ends ofplanar waveguides 720 a and 720 b, the separation of the GRIN fibersegments 220 a and 220 b, the thickness and index of the opticalcomponent 300, and the thickness and index of any medium between theGRIN fiber segments and the optical component 300). Prior to, during,and/or after fabrication of planar waveguides 720 a and 720 b onsubstrate 721, a V-groove is provided on substrate 721 between thewaveguides. Spatially selective material processing of substrate 721(wafer scale, bar level, or individual chip level) enables substantiallycoaxial alignment of waveguides 720 a and 720 b (coaxial with respect totheir respective optical modes), and alignment of the V-groove withrespect to the waveguides 720 a and 720 b. This relative alignment ofthe V-groove and waveguides enables substantially coaxial opticalalignment of the planar waveguides 720 a and 720 b and GRIN fibersegments 220 a and 220 b positioned in the V-groove. Slots or grooves723 a and 723 b may be formed for removing any sloped ends of theV-groove that may be present near the proximal ends of waveguides 720 aand 720 b, and may be formed by any suitable spatially selectivematerial processing step(s). A recessed portion 724 is formed onsubstrate 721 between waveguides 720 a and 720 b by spatially selectivematerial processing, eliminating a central portion of the V-groove anddividing it into V-groove segments 722 a and 722 b.

[0066] Planar waveguide substrate 721 thus provided with planarwaveguides 723 a and 720 b, V-groove segments 722 a and 722 b, andrecessed portion 724 is prepared for receiving thereon a dual-lensoptical assembly 200, including an optical component 300. Opticalassembly substrate 250 is inverted and placed on planar waveguidesubstrate 721 (i.e., “flip-chip” mounted) so that GRIN fiber segments220 a and 220 b are received in corresponding V-groove segments 722 aand 722 b, and optical component 300 is received within recessed portion724. This flip-chip mounting of optical assemblies may be implemented onsubstrate 721 on a wafer scale, bar level, or individual chip level. Thesizes of the GRIN fiber segments 220 a and 220 b, the V-groove segments722 a and 722 b, the optical component 300, and the recessed portion 724are such that the V-groove segments mechanically engage the GRIN fibersegments before the optical component has made contact with bottom orside surfaces of the recessed portion. Such engagement results insubstantially coaxial optical alignment of GRIN fiber segments 220 a and220 b with corresponding planar waveguides 720 a and 720 b (withinoperationally acceptable tolerances).

[0067] The distance between the outer edges of slots 723 a and 723 b maybe wider than the separation between the outer end faces of the GRINfiber segments, and waveguide substrate 721, optical assembly substrate250, V-groove segments 722 a and/or 722 b, recessed portion 724, and/oroptical component 300 may be suitably adapted for enabling sufficientlyaccurate longitudinal positioning of optical assembly 200 and opticalcomponent 300 between waveguides 720 a and 720 b. Suitable adaptationsmay include, but are not limited to: alignment edges and/or otherregistering surfaces; interlocking surfaces; alignment marks or targets;kinematic alignment structures (such as grooves, recesses, protrusions,and the like); and so on. Alternatively, the outer end face of one ofthe GRIN fiber segments may be positioned against the adjacent outeredge of one of slots 723 a and 723 b, thereby reducing or substantiallyeliminating variations of longitudinal position of optical assembly 200and optical component 300 between waveguides 720 a and 720 b.Alternatively, the distance between the outer edges of slots 723 a and723 b may be only just wide enough (within fabrication tolerances) toaccommodate the distance between outer end faces of the GRIN fibersegments of optical assembly 200, thereby reducing or substantiallyeliminating variations of longitudinal position of optical assembly 200and optical component 300 between waveguides 720 a and 720 b.

[0068] Once properly positioned, optical assembly 200 and opticalcomponent 300 may be secured to planar waveguide substrate 721 betweenplanar waveguides 723 a and 720 b by any suitable means, including butnot limited to: adhesives; embedding media (including encapsulants,polymers, and so forth); one or more clamps, retainers, covers or lids,or other structural components; solder; electrostatic interactions;heat/pressure bonding; and so forth. An index-matching embedding medium(such as a polymer, for example) may be employed for substantiallyfilling optical paths between the proximal end faces of planarwaveguides 720 a and 720 b and the distal end faces of respective GRINfiber segments 220 a and 220 b to reduce or substantially eliminateunwanted reflections and increase overall optical throughput. Such anindex-matching medium may have an index near or between the indices ofthe GRIN fiber segments and the planar waveguide, although any mediumthat reduces the reflectivity at the various optical surfaces could beemployed. The index-matching embedding medium may be the same as thatemployed (if any) between proximal end faces of the GRIN fiber segmentsand the end faces of optical component 300, or may be a different indexmatching medium. The index-matching embedding medium may also serve tosecure the GRIN fiber segments within the respective V-groove segments722 a and 722 b. The index-matching embedding medium may be appliedselectively at the end faces and/or V-grooves, or may be employed as anencapsulant 754 for optical component 300, GRIN lenses 220 a and 220 b,and adjacent portions of planar waveguide 720 a and 720 b (as in FIG.6C; encapsulant may also serve as a hermetic sealant). An index-matchingembedding medium or encapsulant may also function as a mechanical,moisture, chemical, and/or hermetic protective barrier.

[0069] Spatially selective material processing employed for formingwaveguides 723 a and 720 b and V-groove segments 722 a and 722 b enablessufficiently accurate and substantially reproducible/repeatabletransverse and longitudinal alignment (i.e., substantially coaxialoptical alignment) between planar optical waveguides 720 a and 720 b andthe respective GRIN fiber segments 220 a and 220 b. The optical modesare approximately indicated by the dashed lines in FIGS. 6A and 6B. GRINmultimode optical fiber is an extraordinarily well-characterizedcommercial product, and the outer diameter of the fiber may be quiteaccurately known. Spatially selective material processing of varioussubstrates, particularly semiconductor substrates such as silicon,gallium arsenide, indium phosphide, and others, readily attainssub-micron dimensional tolerances. By placing GRIN fiber segments 220 aand 220 b in a common substrate groove formed by spatially selectivematerial processing along with the planar waveguides 720 a and 720 b,the planar waveguides and the GRIN fiber segments can be transverselypositioned sufficiently accurately for enabling an operationallyacceptable degree of optical power transfer. By aligning the end of oneor both of the GRIN fiber segments against a slot edge or employingother alignment feature(s), the GRIN fiber segments can belongitudinally positioned sufficiently accurately for enabling anoperationally acceptable degree of optical power transfer.

[0070]FIGS. 7A-7B illustrate an exemplary dual-lens optical assemblymounted on a planar waveguide substrate 921 between a first planaroptical waveguide 920 a (formed on substrate 921) and a single-modeoptical fiber 920 b (positioned in a V-groove 922 on substrate 921 andsubstantially coaxially optically aligned with waveguide 920 a). Thedual-lens assembly is adapted for transferring optical power between theplanar waveguide and the optical fiber through optical component 300.The GRIN fiber segments 220 a and 220 b are of lengths suitable forsubstantially mode matching (at an operationally acceptable level) anoptical mode transmitted through the proximal end face of planarwaveguide 920 a and an optical mode transmitted through the proximal endface of single-mode optical fiber 920 b (given the separation of theends of waveguide 920 a and fiber 920 b, the separation of the GRINfiber segments 220 a and 220 b, the thickness and index of the opticalcomponent 300, and the thickness and index of any medium between theGRIN fiber segments and the optical component 300). Prior to, during,and/or after fabrication of planar waveguide 920 a on substrate 921,V-groove 922 is provided on substrate 921. Spatially selective materialprocessing of substrate 921 (wafer scale, bar level, or individual chiplevel) enables alignment of V-groove 922 with respect to waveguide 920a. This relative alignment of the V-groove and waveguide enablessubstantially coaxial optical alignment of waveguide 920 a, thesingle-mode optical fiber 920 b positioned in the V-groove 922, and GRINsegments 220 a and 220 b positioned in the V-groove. Any sloped end ofV-groove 922 near the proximal end of waveguide 920 a may be removed byforming slot 923. A recessed portion 924 is formed on substrate 921 nearthe end of waveguide 920 a by spatially selective material processing,eliminating a portion of V-groove 922 and forming V-groove segment 922a.

[0071] Planar waveguide substrate 921 thus provided with planarwaveguide 920 a, V-groove 922 and V-groove segment 922 a, and recessedportion 924 is prepared for receiving thereon a dual-lens opticalassembly 200, including an optical component 300. Optical assemblysubstrate 250 is inverted and placed on planar waveguide substrate 921(i.e., “flip-chip” mounted) so that GRIN fiber segments 220 a and 220 bare received in corresponding V-groove segment 922 a and V-groove 922,and optical component 300 is received within recessed portion 924. Thisflip-chip mounting of optical assemblies may be implemented substrate921 on a wafer scale, bar level, or individual chip level. The sizes ofthe GRIN fiber segments 220 a and 220 b, the V-groove 922 and V-groovesegment 922 a, the optical component 300, and the recessed portion 924are such that the V-groove and V-groove segment mechanically engage theGRIN fiber segments before the optical component has made contact withbottom or side surfaces of the recessed portion. Such engagement resultsin substantially coaxial optical alignment of GRIN fiber segment 220 awith planar waveguide 920 a (within operationally acceptabletolerances). Single-mode fiber 920 b is positioned in V-groove 922,resulting in substantially coaxial optical (and mechanical) alignmentwith GRIN fiber segment 220 b (within operationally acceptabletolerances).

[0072] Waveguide substrate 921, optical assembly substrate 250, V-groove922, V-groove segment 922 a, recessed portion 924, and/or opticalcomponent 300 may be suitably adapted for enabling sufficiently accuratelongitudinal positioning of optical assembly 200 and optical component300 relative to the end of waveguide 920 a. Suitable adaptations mayinclude, but are not limited to: alignment edges and/or otherregistering surfaces; interlocking surfaces; alignment marks or targets;kinematic alignment structures (such as grooves, recesses, protrusions,and the like); and so on. Alternatively, optical assembly 200 (andoptical component 300) may be pushed along the V-groove 922 and V-groovesegment 922 a to butt up against the edge of slot 923, thereby reducingor substantially eliminating variations of longitudinal position ofoptical assembly 200 and optical component 300 relative to the end ofwaveguide 920 a. For longitudinal positioning of single mode fiber 920b, alignment stops and/or alignment marks (none shown) may be formedwithin or near the groove 922. The proximal end face of single modefiber 920 b may butt against corresponding alignment stops and/or bealigned with corresponding alignment marks for sufficiently accuratelongitudinal positioning relative to the respective GRIN fiber segmentwithout making contact therewith. Alternatively, the proximal end faceof single mode fiber 920 b may butt directly against the distal end faceof the corresponding GRIN fiber segment 220 b for longitudinalpositioning.

[0073] Once properly positioned, optical assembly 200 (with opticalcomponent 300) and single mode optical fiber 920 b may be secured toplanar waveguide substrate 921 by any suitable means, including but notlimited to: adhesives; embedding media (including encapsulants,polymers, and so forth); one or more clamps, retainers, covers or lids,or other structural components; solder; electrostatic interactions;heat/pressure bonding; and so forth. An index-matching embedding medium(such as a polymer, for example) may be employed between the proximalend face(s) of planar waveguide 920 a and/or single mode fiber 920 b andthe distal end faces of respective GRIN fiber segments 220 a and 220 b,to reduce or substantially eliminate unwanted reflections and increaseoverall optical throughput. Such an index-matching medium may have anindex near or between the indices of the GRIN fiber segments, the singlemode fiber, and/or the planar waveguide, although any medium thatreduces the reflectivity at the various optical surfaces could beemployed. The index-matching embedding medium may be the same as thatemployed (if any) between proximal end faces of the GRIN fiber segmentsand the end faces of optical component 300, or may be a different indexmatching medium. The index-matching embedding medium may also serve tosecure the GRIN fiber segments and/or single-mode fiber within theV-groove and/or V-groove segment. The index-matching embedding mediummay be applied selectively at the end faces and/or V-grooves, or may beemployed as an encapsulant 954 for optical component 300, GRIN lenses220 a and 220 b, and adjacent portions of planar waveguide 920 a andsingle-mode fiber 920 b (as in FIG. 7C; encapsulant may also serve as ahermetic sealant). An index-matching embedding medium or encapsulant mayalso function as a mechanical, moisture, chemical, and/or hermeticprotective barrier.

[0074] Spatially selective material processing employed for formingwaveguide 920 a, V-groove 922, and V-groove segment 922 a enablessufficiently accurate and substantially reproducible/repeatabletransverse and longitudinal alignment between planar optical waveguide920 a, single-mode optical fiber 920 b, and the respective GRIN fibersegments 220 a and 220 b, as already described hereinabove. The opticalmodes are approximately indicated by the dashed lines in FIGS. 7A and7B.

[0075] Dual-lens optical assemblies described thus far have involvedsubstantially coaxial optical alignment of waveguides, fibers inV-grooves, fiber segments in V-grooves, and so forth. Suchconfigurations may be appropriate in circumstances in whichsubstantially normal incidence on optical component 300 is suitableand/or acceptable. However, in many instances substantially normalincidence on optical component 300 may lead to undesirableback-reflections, potentially degrading the performance of the opticalsystem through unacceptably high levels of optical feedback. This may bereduced by application of anti-reflection coatings and/or index-matchingmedia, and/or by alignment of the optical component 300 somewhat awayfrom normal incidence (typically by less than about 10°, often less thanabout 5°, perhaps less than about 2°; depends on geometry, modecharacteristics, sensitivity of upstream optical components toback-reflections, and so on). However, off-normal incidence also resultsin lateral displacement of an optical beam propagating through theoptical component (up to a few tens of microns for optical componentsranging from several hundred microns up to a few millimeters long). Suchlateral displacement would result in decreased transmission through adual-lens optical assembly as described herein if the GRIN fibersegments are substantially collinear. Therefore, additional embodimentsof the optical assemblies disclosed herein may be constructed in whichthe GRIN fiber segments 220 a and 220 b are laterally displaced, and thecorresponding V-groove segments and/or waveguide(s) on a matingfiber/waveguide substrate or waveguide substrate are similarlydisplaced. The optical assembly substrate 250 (including recessed area251, saw cut 254, and/or recessed area 255, as the case may be) may beadapted for aligning the optical component 300 at a well-characterizedoff-normal angle of incidence, enabling precise calculation of theresulting lateral displacement of a transmitted optical beam.

[0076] For forming laterally-displaced dual-lens optical assembliesanalogous to the exemplary substantially collinear embodiments disclosedhereinabove, V-groove(s) 252 may be formed on substrate 250 to comprisemultiple longitudinal segments, laterally displaced from one another bythe calculated distance (FIG. 10). The calculated distance may berealized with the accuracy/precision characteristic of the spatiallyselective material processing employed therefor. Recessed areas (251 or255, as the case may be) are formed between these laterally displacedsegments of the V-groove 252, regardless of whether cleaving or saw cutsare to be employed for dividing the GRIN fiber into segments. A lengthof GRIN fiber 220 is positioned and secured within the V-groove 252,curving over the recessed areas to accommodate the lateral displacementof the segments of the V-groove 252. Upon dividing the GRIN fiber 220into segments 220 a and 220 b (by cleaving, saw cuts, or other method),the segments are laterally displaced by the same distance as thelaterally displaced V-groove segments 252 a and 252 b. Optical powercollected by one of the GRIN fiber segments and transmitted through anoff-normal optical component 300 positioned between the GRIN fibersegments is therefore substantially centered on the other GRIN segment.Instead of spanning recessed area 251 with a single GRIN optical fiber220 (as shown in the first step of FIG. 10), separate segments of GRINoptical fiber may be positioned and secured with the laterally displacedV-groove segments, and then cleaved or cut to the desired length.

[0077]FIG. 11 shows an exemplary dual-lens optical assembly analogous tothat depicted in FIGS. 5A-5C adapted for transferring optical powerbetween two optical fibers 520 a and 520 b. Dashed lines indicating theoptical modes are omitted. The embodiment of FIG. 11 differs from thatof FIGS. 5A-5C only in that the V-groove segments 252 a/252 b and 552a/552 b are laterally displaced, so as to accommodate an off-normaloptical component 300. Spatially selective material processing may beemployed for forming the various grooves/segments on substrate 250 inthe proper positions. FIGS. 12 and 13 show exemplary optical assembliesanalogous to those depicted respectively in FIGS. 6A-6C (adapted fortransferring optical power between two planar waveguides) and 7A-7C(adapted from transferring optical power between a planar waveguide andan optical fiber). Dashed lines indicating the optical modes areomitted. The embodiments of FIGS. 12 and 13 differ from their respectivecollinear analogues only in that the waveguides 720 a/720 b and V-groovesegments 722 a/722 b (FIG. 12) and the waveguide/fiber 920 a/920 b andV-groove/segment 922/922 a (FIG. 13) are laterally displaced, so as toaccommodate an optical assembly 100 with substantially similarlydisplaced GRIN segments 220 a/220 b and an off-normal optical component300. Spatially selective material processing may be employed for formingthe various waveguide(s), groove(s), and groove segment(s) on substrate721 (FIG. 12) or 921 (FIG. 13) in the proper positions.

[0078] It should be noted that embodiments may be made that areanalogous to the exemplary embodiments of FIGS. 10-13, except that thedisplacement of the GRIN fiber segments is in the vertical dimension.Such embodiments fall within the scope of the present disclosure. Suchvertical displacement may be achieved in a variety of suitable ways,including but not limited to the following examples. Portions ofV-groove 252 may be formed at different depths, so that the resultingV-groove segments 252 a/252 b and the GRIN fiber segments 220 a/220 btherein are at different depths. Corresponding V-groove segments 552a/552 b (if present) would also be formed at different depths. V-groovesegments 722 a/722 b on a waveguide substrate 721, as well as waveguides720 a/720 b, would be formed at differing heights. V-groove segments 922a 922 b, V-groove 922, and waveguide 920 a would be formed on awaveguide substrate 921 at differing heights. Optical component 300,substrate 250, and/or substrate 721 or 921, would be adapted forproviding off-normal faces tilted in the vertical direction.

[0079] It may be desirable in some instances of embodiments similar tothose of FIGS. 6A-6C, 7A-7C, 12, and 13 to position and align theoptical component 300 on the planar waveguide substrate (721 or 921),instead of on substrate 250 (with the GRIN segments 220 a and 220 b).Since transverse positioning of the optical component 300 typically doesnot require a high degree of accuracy, it may not be necessary toinclude the optical component 300 in an optical assembly with the GRINfiber segments. Exemplary assembly procedures are illustrated in FIGS.14 and 15, wherein the optical component 300 is positioned and alignedon waveguide substrate 721 or 921 (the waveguide substrate being adaptedtherefor; adaptations not shown), and then an optical assembly asvariously described hereinabove (minus the optical component 300 butincluding V-groove-mounted GRIN segments 220 a/220 b) is then positionedand aligned on substrate 721 or 921. Dashed lines indicating the opticalmodes are omitted.

[0080] It may be desirable in various disclosed embodiments to positioneach of the GRIN segments 220 a/220 b individually on waveguidesubstrate 721 or 921, or on a second substrate with optical fibers 520 aand 520 b. Instead of forming dual-lens optical assemblies, thesubstrate wafer 250 is divided into individual lens substrates 250 a or250 b, each having thereon only one GRIN segment 220 a or 220 b in acorresponding V-groove segment 252 a/252 b (FIG. 16). GRIN segments 220a and 220 b thus formed may be substantially identical, or may differ inlength. One or more of these single GRIN lens substrates may beassembled onto a waveguide substrate 721 or 921 (adapted in any of theways variously set forth hereinabove) along with an optical component300 as shown in FIGS. 17 and 18. Dashed lines indicating the opticalmodes are omitted. Separate mounting of the two GRIN lens segments maybe used to implement any dual-lens embodiment set forth herein orfalling within the scope of the present disclosure. Embedding and/orindex-matching media or encapsulant(s) may be employed, as describedabove. Alignment and securing of GRIN fiber segments to the substratemay be accomplished as described hereinabove. If GRIN segments arisingfrom adjacent portions of a common GRIN fiber are used, axial asymmetryof the GRIN fiber is self-correlated, as with the dual-lens assembliesdescribed hereinabove. Alternatively, use of separate GRIN segmentsenables use of differing GRIN profiles in a two-lens optical device,adding another design parameter for implementing such two-lens opticaldevices.

[0081] There may be instances in which a single lens may provide anadequate (i.e., operationally acceptable) level of optical powertransfer between fiber(s) and/or waveguide(s). In such cases a singleGRIN segment mounted on its own substrate (as in FIGS. 16-18) may beassembled with other optical components in a manner to that alreadydescribed hereinabove. Exemplary single-lens embodiments are shown inFIGS. 27-29, and fall within the scope of the present disclosure and/orappended claims.

[0082] Various structural adaptations of optical assemblies disclosedherein may be required depending on the intended alignmentconfiguration. Similarly, fabrication and/or assembly procedures mayrequire modifications depending on the alignment configuration to beemployed. In particular, the manner in which GRIN optical fiber 220 issecured to substrate 250 during and after forming GRIN fiber segments220 a and 220 b may vary depending on the subsequent assembly and/or useof the optical assembly. Any suitable method may be employed forsecuring the GRIN fiber 220 once it is positioned in V-groove 252, andfor holding it in place as the fiber is divided into GRIN fiber segments220 a and 220 b, by cleaving, saw cuts, and/or other suitable technique.Suitable methods may include but are not limited to: adhesives;embedding media (including encapsulants, polymers, and so forth); one ormore clamps, retainers, covers or lids, or other structural components;solder; electrostatic interactions; heat/pressure bonding; and so forth.Such methods may be similarly employed for securing GRIN fiber segmentsof an optical assembly into V-groove segments on a planar waveguidesubstrate (as in FIGS. 6A-6C and 7A-7C, for example), or for securingoptical fiber(s) onto a substrate along with the GRIN segments (as inFIGS. 5A-5C, for example).

[0083] Some of these techniques for securing GRIN optical fiber 220 inV-groove 252 may rely on the presence of metal. A metal-coated GRINoptical fiber may be secured within a V-groove using solder, forexample. Such solder may be spatially-selectively applied to thesubstrate 250 and/or V-groove 252, and may be bonded to a metal coatingof a GRIN optical fiber. Solder reflow may enable sufficiently accuratepositioning of the GRIN optical fiber engaged with the V-groove. Anothertechnique relying on the presence of metal is heat/pressure bonding ofglass and/or silicon with aluminum (or perhaps an oxide coating thereof;the exact mechanism is not known). Mechanical bonding of convex surfacesto flat surfaces, wherein one surface is silica or silicon and the othersurface is aluminum, has been previously disclosed in U.S. Pat. No.5,178,319 to Coucoulas and U.S. Pat. No. 5,389,193 to Coucoulas et al.Aluminum coatings maybe applied to substrates and/or optical fibers withaccurately known thicknesses, and the heat/pressure bonding results inno substantial deformation of the substrate, fiber, or coating.Heat/pressure bonding therefore may be employed while maintainingsufficiently accurate positioning of fibers/segments within V-grooves,for example. Substrates 250, 721, and/or 921 may comprise siliconsubstrates, while GRIN fiber 220 may be silica or silica-based.

[0084] In one heat/pressure technique, the GRIN optical fiber may beprovided with an aluminum coating, ranging in thickness from a fewmicrons up to a few tens of microns, typically around 10 microns thick.By applying pressure and heat (or acoustic energy) to the aluminizedGRIN fiber within the V-groove (in silicon substrate 250), a robustmechanical bond is formed between the fiber and the V-groove. Pressure,heat, and/or acoustic energy may be applied along substantially theentire length of GRIN fiber 220 within V-groove 252, or may be appliedin a localized fashion only to those fiber/V-groove segments where aGRIN fiber segment is ultimately desired. After the optical assembly isformed (as in the exemplary procedures of FIGS. 3A-3C, FIGS. 3D-3F, FIG.10, or FIG. 16, with the GRIN segments held in place by thesilicon/aluminum bond; aluminum coatings not shown), heat/pressurebonding may be further employed to assemble it with a silicon planarwaveguide substrate (as in exemplary procedures of FIGS. 6A-6C, FIGS.7A-7C, FIGS. 12-15, FIGS. 17-18, or FIGS. 27-29; aluminum coatings notshown). The aluminized GRIN segments may be bonded to the V-grooves ofthe waveguide substrate by additional application of pressure and heat(or acoustic energy). Since the V-grooves provide both lateral as wellas (with the GRIN fiber diameter) vertical alignment, the additionalsize of the GRIN fiber added by the presence of the aluminum coatingmust be accounted for in designing the V-grooves and waveguides on thewaveguide substrate (721 or 921). Similarly, for fabricating anembodiment analogous to the examples of FIGS. 5A-5C and FIG. 11,aluminized optical fiber (having substantially the same diameter) may beused for fibers 520 a/520 b as well as the GRIN segments. Alternatively,if non-metallized optical fiber is used for fibers 520 a and/or 520 b,then V-groove segments 252 a/252 b and 552 a/552 b must be made ofsuitably differing depths, to achieve an acceptable degree of verticalalignment between the GRIN segments and optical fibers.

[0085] In another heat/pressure technique, non-metallized optical fiberis employed, and a coating of aluminum may be applied to at leastportions of V-groove 252. A silica or silica-based GRIN optical fiber220 pressed into such an aluminized V-groove will form a robustmechanical bond upon application of heat or acoustic energy. It may bedesirable to provide the aluminum coating to only those portions of theV-groove 252 where a GRIN fiber segment is ultimately desired.Alternatively, pressure, heat, and/or acoustic energy may be applied ina localized fashion only to those fiber/V-groove segments where a GRINfiber segment is ultimately desired. Alternatively, substantially theentire V-groove may be aluminized for heat/pressure bonding of GRINoptical fiber 220. Forming recessed areas 251/253 a/253 b as in FIGS.3A-3C or making saw cuts 254/256 a/256 b as in FIGS. 3D-3F (as the casemay be) to divide V-groove 252 into V-groove segments 252 a/252 b alsoserves to remove the aluminum coating and V-groove from regions where itis not needed or desired. After the optical assembly is formed (as inthe exemplary procedures of FIGS. 3A-3C, FIGS. 3D-3F, FIG. 10, or FIG.16, with the GRIN segments held in place by the silicon/aluminum bond;aluminum coatings not shown), heat/pressure bonding may be furtheremployed to assemble it with a silicon planar waveguide substrate (as inthe exemplary procedures of FIGS. 6A-6C, FIGS. 7A-7C, FIGS. 12-15, FIGS.17-18, or FIGS. 27-29; aluminum coatings not shown). The V-groovesegments on the waveguide substrate that engage the GRIN segments may bealuminized, and the GRIN fiber segments of the optical assembly bondedto the V-grooves of the waveguide substrate by additional application ofpressure and heat (or acoustic energy). Since the V-grooves provide bothlateral as well as (with the GRIN fiber diameter) vertical alignment,the thickness of the aluminum coating must be accounted for in designingthe V-grooves and waveguides on the waveguide substrate (721 or 921).Similarly, for fabricating an embodiment analogous to the examples ofFIGS. 5A-5C and FIG. 11, the thickness of the aluminum coating inV-groove segments 220 a/220 b must be accounted for. This may beaccomplished by providing an aluminum coating of substantially the samethickness in V-groove segments 552 a/552 b, by using optical fibers 520a/520 b of suitably differing diameter from the GRIN fiber segmentdiameters, or by forming V-groove segments 252 a/252 b and 552 a/552 bat suitably differing depths, to achieve an acceptable degree ofvertical alignment between the GRIN segments and optical fibers.

[0086] In other exemplary embodiments of optical assemblies andfabrication thereof as disclosed herein, adhesives or polymers may beemployed for securing GRIN fiber 220 within V-groove 252 during andafter forming GRIN fiber segments 220 a/220 b. Such an adhesive orpolymer (these terms shall be used interchangeably hereinbelow) may besubstantially uniformly applied over the substrate wafer. Such anadhesive or polymer may instead be: spatially selectively applied;substantially uniformly applied and spatially selectively removed(before or after curing); or substantially uniformly applied andspatially selectively cured, to facilitate subsequent processing andassembly of the optical component 300 with the optical assembly 200, andto facilitate positioning, alignment, and securing of the opticalassembly 200 relative to other planar waveguide(s) and/or opticalfiber(s). For example, adhesive or polymer may be required to hold GRINfiber 220 during cutting and/or to retain GRIN fiber segments 220 a and220 b properly aligned in V-groove segments 252 a and 252 b. However,any cured adhesive or polymer should be substantially absent fromV-groove segments 552 a and 552 b on substrate 250, in order to enableaccurate alignment of single mode fibers 520 a and 520 b positionedtherein (as in FIGS. 5A-5C or FIG. 11). In another example, any curedadhesive or polymer should be substantially absent from at least aportion of each of GRIN fiber segments 220 a and 220 b to enableaccurate alignment of the optical assembly 200 with GRIN segments 220 aand 220 b engaged with a waveguide substrate V-groove or segment thereof(as in FIGS. 6A-6C, FIGS. 7A-7C, FIGS. 12-15, FIGS. 17-18, or FIGS.27-29).

[0087] Optical assemblies 200 suitable for use as illustrated in FIGS.5A-5C or FIG. 11 may be fabricated according to the exemplary procedureillustrated in FIGS. 8A-8B. A substantially similar procedure may beimplemented for fabricating assemblies on a wafer scale, at the barlevel, or for fabricating individual optical assemblies. A polymer layeris applied in strips 560 substantially perpendicular to V-grooves 252and GRIN fibers 220 positioned therein, thereby leaving interveningsegments of V-grooves 252 substantially free of polymer. Saw cuts 254are made through a central portion of the polymer strips 560, while sawcuts 256 a and 256 b are made along the edges of polymer strips 560. Sawcuts 256 a and 256 b thus positioned allow GRIN fiber segments in theintervening portion of V-groove 252 (the portions substantially lackingpolymer coverage) to be removed therefrom. The substrate wafer isdivided between adjacent saw cuts 256 a and 256 b to form individualoptical assemblies 200, leaving empty V-groove segments 552 a and 552 bon each assembly substrate 250 adjacent each GRIN fiber segments 220 aand 220 b. The optical assembly thus formed is thereby prepared forreceiving single mode fibers 520 a and 520 b, as in FIGS. 5A-5C. Polymer560 substantially completely covers lateral surfaces of GRIN fibersegments 220 a and 220 b, and holds the GRIN fiber segments withinV-groove segments 252 a and 252 b. If additional processing of theoptical assembly is needed or desired, polymer 560 may serve as a maskprotecting lateral surfaces of GRIN fiber segments 220 and 220 b fromsuch processing steps. For example, a wet etch might be employed toimprove the optical quality of the transmissive end surfaces of GRINfiber segments 220 a and 220 b (improve the optical quality relative tothat left by the saw cut process), and polymer 560 protects the lateralsurfaces of the GRIN fiber segments from the wet etch. In such a case,the it may be desirable to apply at least some of the polymer(precursor) prior to placement of GRIN fiber 220, to ensure that theunderside of the GRIN fiber segments (within the V-groove segments) areprotected as well. Any embedding medium and/or index matching medium orencapsulant employed with optical component 300 and/or single-modefibers 520 a and 520 b (as in FIG. 5C) may also surround polymer 560.

[0088] Optical assemblies 200 suitable for use as illustrated in FIGS.6A-6C, FIGS. 7A-7C, FIGS. 12-15, FIGS. 17-18, or FIGS. 27-29 may befabricated according to the exemplary procedure illustrated in FIGS.9A-9B. A substantially similar procedure may be implemented forfabricating assemblies on a wafer scale, at the bar level, or forfabricating individual optical assemblies. A polymer layer is providedas sets of three strips 754/756 a/756 b substantially perpendicular toV-grooves 252 and GRIN fibers 220 positioned therein. Polymer strips754/756 a/756 b are positioned to correspond roughly to the positions ofsaw cuts 254/256 a/256 b, and the intervening segments of GRIN opticalfiber 252 are substantially free of polymer. Polymer strip 754 is widerthan saw cut 254, which divides polymer strip 754 into polymer stripsegments 754 a and 754 b. Polymer strips 756 a and 756 b are positionedso that portions of their respective outer edges are removed by saw cuts256 a and 256 b. As a result, GRIN fiber segment 223 a is secured withinV-groove segment 252 a by a polymer strip 754 a at its inner (i.e.,proximal) end and polymer strip 756 a at its outer (i.e., distal) end,leaving a central portion of the GRIN fiber segment substantially freeof polymer. GRIN fiber segment 220 b is similarly secured withinV-groove segment 252 b by a polymer strip 754 b at its inner end andpolymer strip 756 b at its outer end, leaving a central portion of theGRIN fiber segment substantially free of polymer. The substrate wafer isdivided between adjacent saw cuts 256 a and 256 b to form individualoptical assemblies 200. If additional processing is needed or desired(such as a wet etch to improve optical quality of GRIN fiber segmenttransmissive end surfaces, as described hereinabove), a temporary mask(not shown) may be employed, if needed, to protect the exposed lateralsurfaces of the GRIN fiber segments. Such a temporary mask is typicallyselectively removable from the GRIN fiber segments while leaving polymerstrips 754/756 a/756 b intact.

[0089] The substantially polymer-free central portions of the GRIN fibersegments facilitate accurate vertical alignment of the GRIN fibersegments within V-grooves and/or V-groove segments on the waveguidesubstrate. The polymer strips 756 a/756 b may be received within slotsin the waveguide substrate (to enable engagement of the GRIN fibersegments with the waveguide substrate V-groove; slots 723 a/723 b inFIGS. 6A-6C; slot 923 in FIGS. 7A-7C along with an additional slot notshown). The polymer strip segments 754 a/754 b may be similarly receivedwithin a recessed portion of the waveguide substrate along with opticalcomponent 300 (recessed portion 724 in FIGS. 6A-6C; recessed portion 924in FIGS. 7A-7C). One or more of polymer strips 754 a/754 b/756 a/756 bmay engage a substrate for facilitating longitudinal alignment of theGRIN segment(s). An optical assembly thus formed may be employed betweentwo planar optical waveguides (as in FIGS. 6A-6C, FIG. 12, FIG. 14, FIG.17, or FIG. 27) or between a planar optical waveguide and a single-modeoptical fiber (as in FIGS. 7A-7C, FIG. 13, FIG, 15, FIG. 18, FIG. 28, orFIG. 29). Any embedding medium and/or index matching medium orencapsulant employed with optical component 300, planar waveguide(s),and/or a single-mode fiber may also surround polymer strips 754 a/754b/756 a/756 b.

[0090] Another exemplary procedure for forming optical assembly 200which would leave portions of GRIN fiber segments substantially free ofcured polymer is shown in FIG. 19. A substantially uniform polymer oradhesive layer 999 is deposited on substrate wafer 250, which includesV-groove(s) 252. GRIN optical fiber(s) 220 are positioned in V-groove(s)252, and the polymer or adhesive is cured. The polymer layer 999 issufficiently thin that when GRIN optical fibers are placed in thepolymer layer and positioned in the V-groove(s), the polymer leaves asignificant portion of the circumference of the GRIN fiber uncovered.The uncovered portion of the circumference is large enough to allow itto engage the corresponding V-groove on a waveguide substrate uponassembly therewith. Saw cuts 254 (shown in FIG. 19) and 256 a/256 b (notshown in FIG. 19) are made as described hereinabove, optical component300 positioned and the substrate wafer divided into individual assemblysubstrates, and the resulting optical assemblies assembled ontoV-grooves of corresponding waveguide substrates (as in FIGS. 6A-6C and7A-7C). The success of this technique may depend on the viscosity and/orwetting properties of the polymer before curing, which may facilitate orrender problematic the placement of the GRIN fiber while maintaining aportion of its circumference substantially free of polymer.

[0091] It should be understood that any of the techniques describedherein for forming a dual-GRIN-lens assembly may be suitably adapted forforming single-GRIN-segment assembly (i.e., a GRIN segment mounted onits own substrate, as in FIGS. 16-18 and 27-29), and that suchadaptations fall within the scope of the present disclosure and/orappended claims.

[0092] It should be understood that optical propagation through any ofthe optical assemblies disclosed herein may occur in either or bothdirections. In particular, in the exemplary embodiments of FIGS. 7A-7C,FIG. 13, FIG. 15, FIG. 18, and FIGS. 28-29, optical power may propagatefrom planar waveguide 920 a, through optical assembly 200 and opticalcomponent 300, and into single-mode optical fiber 920 b, and also fromsingle-mode optical fiber 920 b, through optical assembly 200 andoptical component 300, and into planar waveguide 920 a. It should beunderstood that while exemplary embodiments have been disclosedincluding optical assemblies used with single-mode optical fiber, theoptical assemblies disclosed herein may also be adapted for and usedwith multi-mode optical fiber, expanded-mode optical fiber, and othertypes of optical fiber. Similarly, planar waveguides used with thedisclosed optical assemblies may be single-mode, multi-mode,expanded-mode, and so forth. Such uses of the disclosed opticalassemblies with various types of optical fibers and/or planar waveguidesshall nevertheless fall within the scope of the present disclosureand/or appended claims.

[0093] In addition to the example of an optical isolator as opticalcomponent 300, a dual-lens optical assembly 200 as disclosed herein maybe implemented for placement and alignment of any one or more suitable“free-space” optical component(s) 300 between GRIN fiber segments 220 aand 220 b. Such components may include, but are not limited to:polarization-dependent and/or polarization-independent opticalisolators; thin-film filters; bulk polarizers, polarization rotators,waveplates, birefringent wedges, analyzers, and/or other polarizationcomponents; micro-mirrors and/or micro-beamsplitters; optical taps;diffractive optics; and so forth. A portion of the functionality of anyfree-space optical component 300 (incorporated into an optical assemblyaccording to the present disclosure) may reside in a planar waveguides720 a and/or 720 b (FIGS. 6A-6C, FIG. 12, FIG. 14, FIG. 17, FIG. 27) orwaveguide 920 a (FIGS. 7A-7C, FIG. 13, FIG. 15, FIG. 18, and FIGS.28-29). Such waveguide-based functionality may be implemented on awafer-scale for many waveguides concurrently, and may potentially reducethe size and/or cost of the component 300 and/or the size of the gaprequired between the planar waveguide(s) and/or optical fiber, as wellas potentially reducing the overall manufacturing cost of the finishedoptical assemblies. In one example of such inclusion of functionality ina waveguide, one linear polarizer may be omitted from a conventionaloptical isolator (potentially reducing its length). The required linearpolarization selectivity may instead be incorporated into the planarwaveguide on the substrate (by any suitable means).

[0094] In addition to GRIN optical fiber for forming GRIN fiber segments220 a/220 b, other gradient-index optical media, such as GRIN rods, maybe employed for forming functionally equivalent lens-like components aspart of optical assemblies disclosed herein. Use of such alternativeGRIN optical media shall nevertheless fall within the scope of thepresent disclosure and/or appended claims.

[0095]FIGS. 20A and 20B illustrate another exemplary optical assembly. Aplanar waveguide 1120 is provided on a waveguide substrate 1102.Waveguide 1120 may be provided at its proximal end with a mode-expandersegment 1122. The mode expander is preferably arranged so as to providesubstantially adiabatic mode expansion, i.e., longitudinal variation ofwaveguide dimensions and/or properties are sufficiently gradual so thatonly an operationally acceptable fraction of propagating optical poweris lost by transfer into undesired modes. Optical power propagatingthrough waveguide 1120 and mode expander 1122 exits through an end-facetof the waveguide/mode expander and diverges by so-called free-spacediffraction, determined primarily by the wavelength and the modecharacteristics at the end facet of the waveguide. The optical powerthen enters an entrance face of an optical component 1140 (such as anoptical isolator, for example) and propagates therethrough. Thetransverse dimensions of the optical isolator are typically too large toprovide substantial lateral confinement of the propagating opticalpower, which therefore continues to diverge through the isolator. Afterexiting the isolator, the still-diverging optical power enters aproximal end face of an optical fiber 1170. Optical fiber 1170preferably comprises a single-mode optical fiber provided at itsproximal end with a short segment of multi-mode GRIN optical fiber,which acts as a focusing element 1172 (equivalently, as an expanded-modecoupling segment), and serves to “collect” the diverging optical powerexiting the optical component 1140 and couple a substantial portion ofthe optical power into the single-mode fiber 1170. The length of thesegment 1172 is preferably selected so as to substantially spatiallymode match the diverging optical power entering segment 1172 and themode supported by single-mode optical fiber 1170 (described furtherhereinbelow).

[0096] The exemplary optical assembly of FIGS. 20A and 20B may bemodified in a variety of ways for achieving a variety of performancerequirements and/or optimizing a variety of performance characteristics.The divergence of the optical mode emerging from the exit face ofwaveguide 1120 may be selected by suitable design of mode-expander 1122.For example, if optical component 1140 is relatively long (greater thanabout 0.5 mm, for example; isolators with cemented Polarcor® or otherbulk polarizers may typically exceed this length), it may be desirableto design mode expander 1122 to provide a larger mode size at thewaveguide exit face (and therefore correspondingly smaller modedivergence). Mode sizes greater than about 10 μm up to several tens ofμm across may be desirable. On the other hand, an exiting mode size ofless than about 10 μm may be desirable for enabling direct end-transferof optical power between waveguide/mode expander 1120/1122 and a singlemode optical fiber (butt-coupling or end-coupling), without anexpanded-mode coupling fiber segment and/or without the interveningoptical component. This may be the case when a common planar waveguidesubstrate 1102 is to be manufactured (with one or more waveguidesthereon) that may be used for optical assemblies either including (FIG.21A) or not including (FIG. 21B) an optical component 1140 (with asingle-mode optical fiber in groove 1107 and pushed forward nearwaveguide 1120 when component 1140 is not present, with or without anexpanded-mode coupling segment 1172; shown without segment 1172 in FIG.21B). The smaller exiting mode size and correspondingly largerdivergence may limit the length of an isolator that may be used (toperhaps less than about 0.5 mm long; isolators with thin-film polarizercoatings applied thereto may fall within this length range), may resultin over-filling of the input face of optical fiber 1170, and/or mayrequire a suitably adapted focusing element or expanded-mode couplingsegment 1172 for optical fiber 1170 for optimizing optical powertransfer into the single-mode fiber (discussed further below). On theother hand, decreased optical power transmission through the opticalassembly (due to the larger divergence and possible over-filling of thefiber input face) may be an operationally acceptable consequence of sucha “dual-use” substrate (FIG. 21A).

[0097] Waveguide 1120 and beam expander 1122 may be implemented in avariety of ways. Waveguide 1120 may typically include a core andlower-index cladding. The core may gradually taper (in one or bothtransverse dimensions) along the length of the waveguide until iteventually terminates, leaving only the cladding to function as anair-guided ridge waveguide. Alternatively, the core may taper (eitherdecreasing or increasing in one or both transverse dimensions) withoutterminating to yield a desired expanded mode size. In another suitableconfiguration, a core/cladding waveguide may be embedded in anothermedium or encapsulant having a lower index than the cladding. As thecore gradually tapers away, the cladding begins to act as a waveguidecore, while the embedding medium or encapsulant acts as cladding. Myriadother suitable configurations may be contrived while remaining withinthe scope of the present invention, some of which are disclosed inabove-cited App. No. 60/466,799. If the tapering and/or termination ofthe core are substantially adiabatic, then the core-guided mode willsmoothly evolve into a larger mode supported by the waveguide (anair-guided mode in a ridge waveguide, for example). The spatial modecharacteristics of the core-guided mode (and therefore theconfiguration, parameters, and dimensions of the core/cladding) aretypically dictated by upstream optical devices/components coupled toand/or incorporated into the waveguide 1120. The spatial modecharacteristics of the expanded optical mode (and therefore theconfiguration, parameters, and dimensions of the mode expander) aredetermined by the considerations discussed in the preceding paragraphs.In one of many possible implementations of the present invention, asilica-based waveguide 1120 may include a silicon nitride or siliconoxynitride core, the core being several μm wide and only a few hundrednm thick or less. Such a thin-core waveguide may be well-suited fortransverse-transfer of optical power, as described in U.S. App. Nos.60/334,705, 60/360,261, and 10/187,030, which may be useful for upstreamoptical power transfer into waveguide 1120 for propagation therethrough.Alternatively, a thin core waveguide of this type may enablespatial-mode-matched end-transfer of optical power with various otherwaveguides and/or devices. The width of the thin core may decrease alongthe mode-expander segment 1122 with the core eventually terminating,leaving a larger doped silica core within the cladding, or leaving onlysilica-based cladding material to act as an air-guided ridge waveguideor as a waveguide core within a lower-index embedding medium orencapsulant. Sufficiently gradual tapering and termination of the thincore results in substantially adiabatic evolution of the core-guidedoptical mode into an expanded optical mode. It should be noted that insome implementations of the present invention, a mode expander 1122 maybe completely omitted, so that the optical mode supported by waveguide1120 simply exits the waveguide and propagates therefrom.

[0098] Many other types and/or configurations of planar waveguides maybe equivalently employed for implementing embodiments set forth hereinand/or equivalents thereof. These may include ridge waveguides, buriedwaveguides, semiconductor waveguides, other high-index waveguides,silica-based waveguides, polymer waveguides, other low-index waveguides,core/clad type waveguides, multi-layer reflector waveguides, metal-cladwaveguides, air-guided waveguides, photonic crystal-based or photonicbandgap-based waveguides, and myriad other examples not explicitly setforth herein but that may nevertheless fall within the scope ofinventive concepts disclosed and/or claimed herein. In any of theseexamples, substantially adiabatic variation of one or more of thematerial(s), configuration, parameters, and/or dimensions of thewaveguide may be employed for producing an expanded optical mode exitingthe mode expander with the desired divergence characteristics. It shouldbe noted that a planar waveguide mode expander as described herein maybe formed at the proximal ends of planar waveguides 720 a and/or 720 b(in the exemplary embodiments of FIGS. 6A-6C, FIG. 12, FIG. 14, FIG. 17,or FIG. 27) or planar waveguide 920 a (in the exemplary embodiments ofFIGS. 7A-7C, FIG. 13, FIG, 15, FIG. 18, FIG. 28, or FIG. 29).

[0099] Optical fiber 1170 with expanded-mode coupling segment 1172(equivalently, collecting segment or focusing segment) may include asingle-mode optical fiber and a short segment of multi-mode opticalfiber coupled to its end to serve as a focusing or collecting element(functionally similar to GRIN segment(s) 220 a and/or 220 b of FIGS.5A-5C or FIG. 11, or to GRIN segment 220 b of FIGS. 7A-7C, FIG. 13, FIG.15, or FIG. 18, for example). The index profile of the multi-mode fiberand the length of the segment 1172 may be tailored to optimally couple afreely-propagating optical mode and the mode of single-mode fiber 1170fused to segment 1172. The multi-mode fiber segment may be secured tothe single-mode fiber for end-transfer of optical power (i.e.,butt-coupling or end-coupling) by a variety of means, preferably byfusion splicing of the multi-mode GRIN fiber segment to the single-modefiber, or alternatively by cementing or by a ferrule or other similarmechanical device. Relative concentricity of the multi-mode fibersegment and the single-mode fiber is desirable, and may be readilyachieved by fusion splicing of fibers having substantially similar outerdiameters. Gradient-index multi-mode fiber having various corediameters, cladding diameters, and/or index gradients are availablecommercially, and may be selected for satisfying specific performancerequirements for mode-expanded coupling segment 1172. In particular,GRIN optical fiber having an outer cladding diameter of about 125 μm iswell-suited for substantially co-axial fusion to standard single-modeoptical fiber (which also typically has an outer cladding diameter ofabout 125 μm). Quarter-pitch lengths for available GRIN optical fiberare on the order of 0.2 mm to 2 mm, and segments may be cleaved orpolished to a design length with accuracy of a few microns for achievingdesire focusing properties.

[0100] A single-mode optical fiber 1170 with an expanded-mode couplingsegment 1172 may be substantially coaxially positioned with respect towaveguide 1120 and the mode-expander 1122 thereof, leaving a gap betweenthem for accommodating optical component 1140 (an optical isolator, forexample). The mode expander 1122 and expanded-mode coupling segment 1172may be suitably adapted (as described in the preceding paragraphs) so asto achieve an optimal level of optical power transfer therebetween for aparticular longitudinal spacing. Alternatively, mode expander 1122 andexpanded-mode coupling segment 1172 may be suitably adapted so as toachieve an operationally acceptable level of optical power transfertherebetween over a range of longitudinal spacings (which may include nogap, i.e., butt-coupling of the waveguide 1120 and fiber 1170). Thelongitudinal spacing (or range thereof) may be determined by the size(or range of sizes) of component 1140 that may be placed betweenwaveguide 1120 and fiber 1170.

[0101] It may be desirable to provide waveguide 1120 with anexpanded-mode coupling segment 1122 having performance (i.e., focusingproperties) similar to segment 1172. As shown in FIGS. 22A and 22B,waveguide 1120 may terminate with a segment 1122 with an enlarged core(including an index gradient; parameters determined by requirements forpropagating through the isolator and into the optical fiber), whichwould function in a manner analogous to GRIN fiber segment 1172 coupledto single-mode fiber 1170. Waveguide segment 1122 may be configuredslightly longer than the corresponding quarter-pitch length, therebyproviding a collimated or slightly convergent optical mode forpropagation through isolator 1140. This may enable use of a longeroptical component 1140, since the divergent portion of the optical modeentering segment 1172 will be shifted farther from the exit face ofsegment 1122.

[0102] Transverse and/or angular alignment of waveguide 1120 and fiber1170 significantly affects the degree of optical power transfer that maybe achieved therebetween. Use of a planar waveguide on a substrate mayfacilitate transverse alignment, and may enable passive assembly ofwaveguide, isolator, and optical fiber while achieving operationallyacceptable levels of optical power transfer through the opticalassembly. In particular, fabrication of alignment structures on thewaveguide substrate (for later positioning of the optical component andthe optical fiber) substantially concurrently with fabrication of thewaveguide 1120 and mode expander 1122 (if present), usingspatially-selective fabrication techniques, ensures accurate relativepositioning of the alignment structures relative to the waveguide/modeexpander. In turn, these alignment structures (grooves, V-grooves,pockets, edges, risers, posts, and so on, which may be very accuratelyformed on a silicon PLC substrate using standard lithography techniques,for example), enable accurate placement of the optical component 1140and optical fiber/expanded-mode coupling segment 1170/1172 on substrate1102 relative to waveguide/mode expander 1120/1122. Sub-micron accuracyis readily achieved using standard lithographic techniques on a siliconsubstrate, for example. FIGS. 20A/20B, 21A/21B, and 22A/22B show agroove 1104 for receiving and positioning optical component 1140, and aV-groove 1107 for receiving optical fiber/focusing segment 1170/1172.Groove 1104 may be slightly over-sized (for tolerance purposes), and maybe fabricated so that one edge provides the required longitudinal and/orangular alignment of component 1140. End faces of isolator 1140 maytypically be oriented substantially normal to a propagation axis definedby waveguide 1120 and fiber 1170/groove 1107, or a non-normal angle ofincidence on component 1140 may be desirable for a variety of reasons;in either case the orientation of at least one edge of groove 1104provides the necessary alignment guide.

[0103] Other structures, waveguides, devices, and so forth may also befabricated on substrate 1102 substantially concurrently with thewaveguide/mode expander and alignment structures, for formingmulti-component optical devices and/or assemblies. Furthermore, thesespatially-selective fabrication techniques may typically be implementedon a wafer scale for many devices simultaneously (hundreds or eventhousands of devices on a single wafer). Passive assembly/alignment andwafer-scale processing each contribute significantly for enablingsubstantial economies-of-scale to be realized for the fabrication andassembly of finished optical assemblies and/or devices.

[0104] An optical assembly according to the present invention may beemployed for providing an “in-line” optical isolator for an opticalfiber (FIGS. 23A and 23B). A PLC or other planar waveguide 1120 on awaveguide substrate 1102 may be provided with an distal end adapted forsubstantially spatial-mode-matched end-transfer of optical power with asingle-mode optical fiber 1110. A V-groove 1101 may be provided in thewaveguide substrate 1102 for receiving and aligning the optical fiber1110 relative to the waveguide 1120. Optical power propagating throughthe waveguide diverges from its proximal end face. The proximal end ofthe waveguide 1120 may be suitably adapted for achieving desireddivergence properties for the exiting optical power (with mode expander1122, for example), which then enters the optical component 1140 andpropagates therethrough (still diverging). The optical component 1140may be received in and positioned by an alignment groove 1104 providedin the waveguide substrate 1102. A V-groove 1107 is provided in thewaveguide substrate 1102 for receiving an optical fiber 1170 andaligning it with the waveguide 1120. The proximal end of the opticalfiber 1170 may include an expanded-mode coupling segment 1172 (i.e., afocusing segment comprising a GRIN optical fiber segment fusion-splicedor otherwise joined to the single-mode optical fiber 1170) as describedhereinabove, suitably adapted for collecting optical power transmittedthrough the component 1140 and coupling it into the mode supported bythe optical fiber 1170. Wafer-scale fabrication of many waveguidessubstantially concurrently with the grooves, V-grooves, and any otherrequired alignment structures may ensure operationally acceptableoptical power throughput for passively assembled and aligned devices(while also reducing manufacturing costs). Additional components may beimplemented on the waveguide substrate (on a wafer scale) for producinghigh-performance isolators, such as optical tap(s), optical detector(s)for monitoring power, thermo-optic compensator element(s), electro-opticelement(s), and so on. Feedback and/or control circuitry may be employedfor stabilizing variations of isolator performance with wavelength,temperature, and/or other variables.

[0105] Alignment structures on the planar waveguide substrates may beemployed for aligning other mode-expanding and/or focusing opticalelements incorporated into the optical assembly. For example, as shownin FIGS. 24A/24B and 26A/26B, instead of mode expander 1122 integrallyprovided for waveguide 1120, a ball or aspheric lens 1124 may beemployed instead as a mode expander and focusing element for reducingthe divergence through optical component 1140, or even producing asubstantially collimated or convergent optical mode. An alignment pocket1103 may be fabricated in substrate 1102 (once again, substantiallyconcurrently with waveguide 1120) for accurately positioning lens 1124relative to waveguide 1120. Similarly, a ball or aspheric lens 1174 maybe employed as an expanded-mode fiber coupler instead of fiber segment1172, as in FIGS. 24A/24B and 25A/25B. An alignment pocket 1108 may befabricated in substrate 1102 for receiving and positioning lens 1174aligned with waveguide 1120, pocket/lens 1103/1124 (if present), groove1104, and V-groove 1107.

[0106] In another example, as shown in FIGS. 30A/30B and 32A/32B,instead of mode expander 1122 integrally provided for waveguide 1120, adiffractive or Fresnel lens 1126 (a silicon micro-Fresnel lens, forexample) may be employed instead as a mode expander and focusing elementfor reducing the divergence through optical component 1140, or evenproducing a substantially collimated or convergent optical mode. Analignment groove 1105 may be fabricated in substrate 1102 (once again,substantially concurrently with waveguide 1120) for accuratelypositioning Fresnel lens 1126 relative to waveguide 1120. Similarly, aFresnel lens 1176 may be employed as an expanded-mode fiber couplerinstead of fiber segment 1172, as in FIGS. 30A/30B and 31A/31B. Analignment groove 1109 may be fabricated in substrate 1102 for receivingand positioning Fresnel lens 1176 aligned with waveguide 1120,pocket/lens 1105/1126 (if present), groove 1104, and V-groove 1107.

[0107] It should be noted that any desirable “mixing and matching” ofdual-GRIN-segment assembly 200, single GRIN segments 220 a/220 b,ball/aspheric lenses 1124/1174, waveguide mode expanders (such as modeexpander 1122), Fresnel lenses 1126/1176, and/or spliced GRIN segments(such as segment 1172) may be employed to achieve a desired or requiredlevel of optical power transfer between fiber(s) and/or waveguide(s)through an optical component. Such mixed/matched optical assembliesshall fall within the scope of the present disclosure and/or appendedclaims. Suitable index-matching embedding media may be employed forsubstantially filling optical paths in the any of the exemplaryembodiments and/or mixed/matched variants thereof, and/or forencapsulating those embodiments, in a manner already describedhereinabove.

[0108] It should be understood that while propagation of optical powermay often be described herein as proceeding from the waveguide, throughone or more free-space optical components, and into an optical fiber,optical power propagation in the either of both directions shall fallwithin the scope of the present disclosure and/or appended claims.

[0109] It should be noted that the single- and dual-GRIN-lensassemblies, waveguide mode expanders, and fiber- or waveguide-basedfocusing elements (i.e., expanded-mode couplers) as disclosed herein maybe less affected by a surrounding optical medium than previous designsbased on ball lenses. Optical performance of a ball lens dependsexplicitly on the index contrast between the ball and the surroundingmedium, while the GRIN lenses, mode expanders, and focusing elementsdisclosed herein may be designed so as to be substantially unaffected bythe index of the surrounding medium. For this reason, optical assembliesdisclosed herein that include only GRIN lenses, waveguide- orfiber-based focusing elements, and/or waveguide mode expanders may besuitable for embedding or encapsulating in another medium withoutsubstantially altering optical performance characteristics. Suitableembedding media or encapsulants may include dielectrics, index-matchingmedia, polymers (including epoxies), and/or other suitable “potting”media. The potting medium may be chosen for one or more of its:refractive index, power-handling capability, hermetic sealing properties(such as low out-gassing and/or hydrophobicity, for example), flowproperties, surface tension, optical homogeneity, and so forth. Gapsbetween transmissive end faces within the optical assembly, whetherintentional (for tolerance purposes or for accommodating a variety ofcomponent lengths) or unintentional, may be filled with the embeddingmedium or encapsulant. In addition to maintaining alignment of theembedded components and protecting sensitive optical surfaces thereof,this may serve to substantially reduce Fresnel losses at thecorresponding optical interfaces, may serve to reduce the divergence ofthe propagating optical power between the waveguide and optical fiber,and may also serve to reduce the dependence of overall transmission ofthe optical assembly on unintended angular deviations of entrance/exitfaces of waveguide 1120, component 1140, and/or optical fiber 1170.

[0110] An optical Isolator as component 1140 typically includes aFaraday rotator crystal configured for non-reciprocal 45° rotationplaced between a pair of linear polarizers with their transmission axesoffset by 45°. These may be Polarcor® or other bulk polarizers cementedor otherwise secured to the faces of the Faraday rotator, or may beprovided as thin film coatings on the faces of the Faraday rotator.Alternatively, part of the functionality of the isolator may beincorporated into waveguide 1120 (as already described hereinabove forwaveguide(s) 720 a/720 b/920 a; the following discussion shall alsoapply to those exemplary embodiments). In particular, it may bedesirable for a portion of waveguide 1120 to function as apolarization-selective element, either by exploiting its intrinsicproperties or by providing a polarization-selective structure, such as abend in the waveguide or a coating thereon, for example. In anotherexample, waveguide 1120 may be adapted for transverse-transfer ofoptical power (either mode-interference-coupled or substantiallyadiabatic transverse-transfer; as disclosed in U.S. App. No. 60/334,705,U.S. App. No. 60/360,261, and U.S. application Ser. No. 10/187,030, forexample) from an upstream optical device into waveguide 1120 and thenceinto fiber 1170. Transverse-transfer of optical power into waveguide1120 may be made polarization selective, so that the transverse-transfersegment of waveguide 1120 (i.e., the optical junction segment) may takethe place of the first polarizer of isolator 1140. Any such segment ofwaveguide 1120 configured for substantial transmission of only onelinear polarization may take the place of the first polarizer ofisolator 1140. In such embodiments (wherein a portion of the isolatorfunctionality, i.e., the first polarizer, is incorporated into waveguide1120), the gap between waveguide 1120 and fiber 1170 may be reduced,thereby easing the requirements for efficient optical power transfertherebetween. Incorporation of the first isolator polarizer intowaveguide 1120 may also serve to reduce overall cost for the opticalassembly, since the polarization-selectivity may be incorporated intothe waveguide during the same wafer-scale fabrication sequence used forits fabrication, and therefore adds negligibly to the overallmanufacturing cost of the finished optical assemblies. The cost ofisolator 1140 would also be reduced by the omission of the firstpolarizer thereof, while potential misalignment of the first polarizerwith respect to the input polarization would be substantially reduced asa source of optical loss.

[0111] Optical assemblies as disclosed herein may be readilyincorporated into higher-level integrated optical devices. In theschematic diagram of FIG. 33, optical output of laser source 3301 (ofany suitable type) is transmitted by transmission optical element 3320 athrough optical assembly 3300 (including an optical component, forexample an optical isolator) into transmission optical element 3320 b.Transmission optical elements 3320 a and 3320 b may include anycombination of planar waveguide(s) and/or optical fiber(s) as disclosedherein. The optical assembly 3300 includes one or more focusing opticalelements and is assembled with transmission elements 3320 a and 3320 b,in a manner similar to any of the exemplary embodiments disclosed hereinor substantially equivalent thereto. In the schematic diagram of FIG.34, an optical transceiver 3401 is optically coupled to transmissionoptical element 3420 b through transmission optical element 3420 a andoptical assembly 3400 (including an optical component, for example anoptical isolator). Transceiver 3401 includes at least one laser source(of any suitable type) and at least one photodetector (of any suitabletype). Transmission optical elements 3420 a and 3420 b may include anycombination of planar waveguide(s) and/or optical fiber(s) as disclosedherein. The optical assembly 3400 includes one or more focusing opticalelements and is assembled with transmission elements 3420 a and 3420 b,in a manner similar to any of the exemplary embodiments disclosed hereinor substantially equivalent thereto. In the schematic diagram of FIG.35, a photodetector 3501 (of any suitable type) is optically coupled totransmission optical element 3520 b through transmission optical element3520 a and optical assembly 3500 (including an optical component, forexample an optical isolator or optical filter). Transmission opticalelements 3520 a and 3520 b may include any combination of planarwaveguide(s) and/or optical fiber(s) as disclosed herein. The opticalassembly 3500 includes one or more focusing optical elements and isassembled with transmission elements 3520 a and 3520 b, in a mannersimilar to any of the exemplary embodiments disclosed herein orsubstantially equivalent thereto.

[0112] In addition to an optical isolator 1140, the present inventionmay be implemented for placement and alignment of any one or moresuitable “free-space” optical components 1140 between a planar waveguide1120 and an optical fiber 1170. Such components may include, but are notlimited to: thin-film filters; bulk polarizers, polarization rotators,waveplates, and/or other polarization components; micro-mirrors and/ormicro-beamsplitters; optical taps; diffractive optics; and so forth. Aswith an isolator, a portion of the functionality of any free-spaceoptical component 1140 (incorporated into an optical assembly accordingto the present invention) may reside in waveguide 1120, incorporatedtherein on a wafer-scale, potentially reducing the size and/or cost ofthe component and/or the size of the gap required between waveguide 1120and fiber 170, as well as potentially reducing the overall manufacturingcost of the finished optical assemblies.

[0113] The term “free-space” optical propagation as used herein shalldenote propagation of optical fields through media which do not providetransverse confinement or guiding of the optical field. Propagation ofoptical fields through such media is determined predominantly bydiffraction. Free-space propagation may occur in vacuum or air, or mayoccur within any substantially transparent medium or a transmissiveoptical component fabricated from substantially transparent material(s).“Substantially transparent” refers to a particular operating wavelengthrange.

[0114] For purposes of the foregoing written description and/or theappended claims, “index” may denote the bulk refractive index of aparticular material (also referred to herein as a “material index”) ormay denote an “effective index” n_(eff), related to the propagationconstant β of a particular optical mode in a particular optical elementby β=2πn_(eff)/λ. The effective index may also be referred to herein asa “modal index”. As referred to herein, the term “low-index” shalldenote any materials and/or optical structures having an index less thanabout 2.5, while “high-index” shall denote any materials and/orstructures having an index greater than about 2.5. Within these bounds,“low-index” may refer to: silica (SiO_(x)), germano-silicate,boro-silicate, other doped silicas, and/or other silica-based materials;silicon nitride (Si_(x)N_(y)) and/or silicon oxynitrides (SiO_(x)N_(y));other glasses; other oxides; various polymers; and/or any other suitableoptical materials having indices below about 2.5. “Low-index” may alsoinclude optical fiber, optical waveguides, planar optical waveguides,and/or any other optical components incorporating such materials and/orexhibiting a modal index below about 2.5. Similarly, “high-index” mayrefer to materials such as semiconductors, IR materials, and/or anyother suitable optical materials having indices greater than about 2.5,and/or optical waveguides of any suitable type incorporating suchmaterial and/or exhibiting a modal index greater than about 2.5. Theterms “low-index” and “high-index” are to be distinguished from theterms “lower-index” and “higher-index”, also employed herein.“Low-index” and “high-index” refer to an absolute numerical value of theindex (greater than or less than about 2.5), while “lower-index” and“higher-index” are relative terms indicating which of two particularmaterials has the larger index, regardless of the absolute numericalvalues of the indices.

[0115] For purposes of the foregoing written description and/or theappended claims, the term “optical waveguide” (or equivalently,“waveguide”) as employed herein shall denote a structure adapted forsupporting one or more optical modes. Such waveguides shall typicallyprovide confinement of a supported optical mode in two transversedimensions while allowing propagation along a longitudinal dimension.The transverse and longitudinal dimensions/directions shall be definedlocally for a curved waveguide; the absolute orientations of thetransverse and longitudinal dimensions may therefore vary along thelength of a curvilinear waveguide, for example. Examples of opticalwaveguides may include, without being limited to, various types ofoptical fiber and various types of planar waveguides. The term “planaroptical waveguide” (or equivalently, “planar waveguide”) as employedherein shall denote any optical waveguide that is formed on asubstantially planar substrate. The longitudinal dimension (i.e., thepropagation dimension) shall be considered substantially parallel to thesubstrate. A transverse dimension substantially parallel to thesubstrate may be referred to as a lateral or horizontal dimension, whilea transverse dimension substantially perpendicular to the substrate maybe referred to as a vertical dimension. Examples of such waveguidesinclude ridge waveguides, buried waveguides, semiconductor waveguides,other high-index waveguides (“high-index” being above about 2.5),silica-based waveguides, polymer waveguides, other low-index waveguides(“low-index” being below about 2.5), core/clad type waveguides,multi-layer reflector (MLR) waveguides, metal-clad waveguides,air-guided waveguides, vacuum-guided waveguides, photonic crystal-basedor photonic bandgap-based waveguides, waveguides incorporatingelectro-optic (EO) and/or electro-absorptive (EA) materials, waveguidesincorporating non-linear-optical (NLO) materials, and myriad otherexamples not explicitly set forth herein which may nevertheless fallwithin the scope of the present disclosure and/or appended claims. Manysuitable substrate materials may be employed, including semiconductor,crystalline, silica or silica-based, other glasses, ceramic, metal, andmyriad other examples not explicitly set forth herein which maynevertheless fall within the scope of the present disclosure and/orappended claims.

[0116] The term “transmission optical element” (equivalently,“transmission element”) as used herein shall denote an optical waveguideprimarily serving to convey optical power from one point to another. Atransmission optical element may serve to alter the optical powertransmitted therethrough, and such alteration may be passive (i.e.,requiring no control signal input) and/or active (i.e., in response toan applied control signal). A transmission optical element shall bedistinguished from an optical source (such as an LED or laser), in thata transmission optical element does not generate optical power, butserves to transmit optical power generated elsewhere.

[0117] One exemplary type of planar optical waveguide that may besuitable for use with optical components disclosed herein is a so-calledPLC waveguide (Planar Lightwave Circuit). Such waveguides typicallycomprise silica or silica-based waveguides (often ridge or buriedwaveguides; other waveguide configuration may also be employed)supported on a substantially planar silicon substrate (often with aninterposed silica or silica-based optical buffer layer). Sets of one ormore such waveguides may be referred to as planar waveguide circuits,optical integrated circuits, or opto-electronic integrated circuits. APLC substrate with one or more PLC waveguides may be readily adapted formounting one or more optical sources, lasers, modulators, and/or otheroptical devices adapted for end-transfer of optical power with asuitably adapted PLC waveguide. A PLC substrate with one or more PLCwaveguides may be readily adapted (according to the teachings of U.S.Patent Application Pub. No. 2003/0081902 and/or U.S. App. No.60/466,799) for mounting one or more optical sources, lasers,modulators, photodetectors, and/or other optical devices adapted fortransverse-transfer of optical power with a suitably adapted PLCwaveguide (mode-interference-coupled, or substantially adiabatic,transverse-transfer; also referred to as transverse-coupling).

[0118] For purposes of the foregoing written description and/or appendedclaims, “spatially-selective material processing techniques” shallencompass epitaxy, layer growth, lithography, photolithography,evaporative deposition, sputtering, vapor deposition, chemical vapordeposition, beam deposition, beam-assisted deposition, ion beamdeposition, ion-beam-assisted deposition, plasma-assisted deposition,wet etching, dry etching, ion etching (including reactive ion etching),ion milling, laser machining, spin deposition, spray-on deposition,electrochemical plating or deposition, electroless plating,photo-resists, UV curing and/or densification, micro-machining usingprecision saws and/or other mechanical cutting/shaping tools, selectivemetallization and/or solder deposition, chemical-mechanical polishingfor planarizing, any other suitable spatially-selective materialprocessing techniques, combinations thereof, and/or functionalequivalents thereof. In particular, it should be noted that any stepinvolving “spatially-selectively providing” a layer or structure mayinvolve either or both of: spatially-selective deposition and/or growth,or substantially uniform deposition and/or growth (over a given area)followed by spatially-selective removal. Any spatially-selectivedeposition, removal, or other process may be a so-called direct-writeprocess, or may be a masked process. It should be noted that any “layer”referred to herein may comprise a substantially homogeneous materiallayer, or may comprise an inhomogeneous set of one or more materialsub-layers. Spatially-selective material processing techniques may beimplemented on a wafer scale for simultaneous fabrication/processing ofmultiple structures on a common substrate wafer.

[0119] It should be noted that various components, elements, structures,and/or layers described herein as “secured to”, “connected to”, “mountedon”, “deposited on”, “formed on”, “positioned on”, etc., a substrate maymake direct contact with the substrate material, or may make contactwith one or more layer(s) and/or other intermediate structure(s) alreadypresent on the substrate, and may therefore be indirectly “secured to”,etc, the substrate.

[0120] The phrase “operationally acceptable” appears herein describinglevels of various performance parameters of optical components and/oroptical devices, such as optical power transfer efficiency(equivalently, optical coupling efficiency), optical loss, optical gain,lasing threshold, undesirable optical mode coupling, and so on. Anoperationally acceptable level may be determined by any relevant set orsubset of applicable constraints and/or requirements arising from theperformance, fabrication, device yield, assembly, testing, availability,cost, supply, demand, and/or other factors surrounding the manufacture,deployment, and/or use of a particular optical device. Such“operationally acceptable” levels of such parameters may therefor varywithin a given class of devices depending on such constraints and/orrequirements. For example, a lower optical coupling efficiency may be anacceptable trade-off for achieving lower optical assembly fabricationcosts in some instances, while higher optical coupling may be requiredin other instances in spite of higher fabrication costs. The“operationally acceptable” coupling efficiency therefore varies betweenthe instances. In another example, higher lasing threshold arising fromoptical loss (due to scattering, absorption, undesirable opticalcoupling, and so on) may be an acceptable trade-off for achieving lowerdevice fabrication cost or smaller device size in some instances, whilea lower lasing threshold may be required in other instances in spite ofhigher fabrication costs and/or larger device size. The “operationallyacceptable” lasing threshold therefore varies between the instances.Many other examples of such trade-offs may be imagined. Opticalassemblies and fabrication methods therefor as disclosed herein, andequivalents thereof, may therefore be implemented within tolerances ofvarying precision depending on such “operationally acceptable”constraints and/or requirements. Phrases such as “substantiallyadiabatic”, “substantially spatial-mode-matched”, “substantiallymodal-index-matched”, “so as to substantially avoid undesirable opticalcoupling”, and so on as used herein shall be construed in light of thisnotion of “operationally acceptable” performance.

[0121] While particular examples have been disclosed herein employingspecific materials and/or material combinations and having particulardimensions and configurations, it should be understood that manymaterials and/or material combinations may be employed in any of avariety of dimensions and/or configurations while remaining within thescope of inventive concepts disclosed and/or claimed herein.

[0122] It is intended that equivalents of the disclosed exemplaryembodiments and methods shall fall within the scope of the presentdisclosure and/or appended claims. It is intended that the disclosedexemplary embodiments and methods, and equivalents thereof, may bemodified while remaining within the scope of the present disclosureand/or appended claims.

What is claimed is:
 1. A method for making an optical apparatus, comprising: forming at least one groove on a substrate; mounting a GRIN optical medium on the substrate with the GRIN optical medium positioned in at least one of the grooves; and dividing the GRIN optical medium to form first and second GRIN segments, the first and second GRIN segments remaining mounted on the substrate in at least one of the grooves, the first and second GRIN segments being substantially parallel and longitudinally spaced apart from one another on the substrate and having respective proximal and distal end faces.
 2. The method of claim 1, further comprising mounting an optical component on the substrate between the proximal ends of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively.
 3. The method of claim 2, further comprising: mounting a first optical fiber on the substrate in one of the grooves optically substantially coaxial with the first GRIN segment; and mounting a second optical fiber on the substrate in one of the grooves optically substantially coaxial with the second GRIN segment.
 4. The method of claim 3, further comprising: substantially filling an optical path between the distal end face of the first GRIN segment and a proximal end face of the first optical fiber with a substantially transparent embedding medium; and substantially filling an optical path between the distal end face of the second GRIN segment and a proximal end face of the second optical fiber with a substantially transparent embedding medium.
 5. The method of claim 2, further comprising: substantially filling an optical path between the first end face of the optical component and the proximal end face of the first GRIN segment with a substantially transparent embedding medium; and substantially filling an optical path between the second end face of the optical component and the proximal end face of the second GRIN segment with a substantially transparent embedding medium.
 6. The method of claim 1, wherein the first and second GRIN segments are optically substantially coaxial, further comprising mounting an optical component on the substrate between the proximal ends of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively, the first and second end faces each being substantially normal to a propagation direction defined by the first and second GRIN segments.
 7. The method of claim 1, wherein the first and second GRIN segments are optically laterally displaced from one another, further comprising mounting an optical component on the substrate between the proximal ends of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively, the first and second end faces each being non-normal with respect to a propagation direction defined by the respective first and second GRIN segments, refraction at the first and second end faces of the optical component defining an optical path through the optical component between the first and second GRIN segments.
 8. The method of claim 1, further comprising: forming at least one groove on a second substrate; mounting the first substrate and the first and second GRIN segments on the second substrate with the first and second GRIN segments in at least one groove of the second substrate; positioning an optical component between the proximal end faces of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively; positioning a first transmission optical element on the second substrate optically substantially coaxial with the first GRIN segment, the first transmission optical element having a proximal end face facing the distal end face of the first GRIN segment; and positioning a second transmission optical element on the second substrate optically substantially coaxial with the second GRIN segment, the second transmission optical element having a proximal end face facing the distal end face of the second GRIN segment.
 9. The method of claim 8, wherein: the first transmission optical element comprises an optical fiber mounted on the second substrate in one of the grooves thereof and optically substantially coaxial with the first GRIN segment; and the second transmission optical element comprises an optical fiber mounted on the second substrate in one of the grooves thereof and optically substantially coaxial with the second GRIN segment.
 10. The method of claim 8, wherein: the first transmission optical element comprises a first planar optical waveguide formed on the second substrate and optically substantially coaxial with the first GRIN segment; and the second transmission optical element comprises a second planar optical waveguide formed on the second substrate and optically substantially coaxial with the second GRIN segment.
 11. The method of claim 8, wherein: the first transmission optical element comprises a planar optical waveguide formed on the second substrate and optically substantially coaxial with the first GRIN segment; and the second transmission optical element comprises an optical fiber secured to the second substrate in one of the grooves and optically substantially coaxial with the second GRIN segment.
 12. The method of claim 8, further comprising: substantially filling an optical path between the first end face of the optical component and the proximal end face of the first GRIN segment with a substantially transparent embedding medium; and substantially filling an optical path between the second end face of the optical component and the proximal end face of the second GRIN segment with a substantially transparent embedding medium.
 13. The method of claim 8, further comprising: substantially filling an optical path between the proximal end face of the first transmission optical element and the distal end face of the first GRIN segment with a substantially transparent embedding medium; and substantially filling an optical path between the proximal end face of the transmission optical element and the distal end face of the second GRIN segment with a substantially transparent embedding medium.
 14. The method of claim 8, further comprising encapsulating the optical component, the first and second GRIN segments, and proximal portions of the first and second transmission optical elements. wherein the optical component is mounted on the first substrate.
 15. The method of claim 8, wherein the optical component is mounted on the first substrate.
 16. The method of claim 8, wherein the optical component is mounted on the second substrate.
 17. The method of claim 8, wherein the optical component comprises an optical isolator.
 18. The method of claim 8, wherein: the optical component comprises a Faraday rotator and a linear polarizer; at least one of the transmission optical elements comprises a polarization-selective planar waveguide formed on the second substrate; and the Faraday rotator, polarizer, and polarization-selective planar waveguide together function as an optical isolator.
 19. The method of claim 8, further comprising optically coupling a laser source to the first transmission optical element at a distal portion thereof so that optical output power from the laser source is transmitted into the second transmission optical element through the first transmission optical element and the optical component.
 20. The method of claim 8, further comprising optically coupling an optical transceiver to the first transmission optical element at a distal portion thereof so that optical power is transmitted between the optical transceiver and the second transmission optical element through the first transmission optical element and the optical component.
 21. The method of claim 8, further comprising optically coupling a photodetector to the first transmission optical element at a distal portion thereof so that optical power is transmitted from the second transmission optical element to the photodetector through the optical component and the first transmission optical element.
 22. The method of claim 1, wherein the GRIN optical medium is a GRIN optical fiber.
 23. An optical apparatus, comprising: a substrate with at least one groove; and first and second segments of GRIN optical medium secured to the substrate in at least one of the grooves, the first and second GRIN segments being substantially parallel and longitudinally spaced apart from one another on the substrate and having respective proximal and distal end faces.
 24. The apparatus of claim 23, further comprising an optical component mounted on the substrate between the proximal ends of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively.
 25. The apparatus of claim 24, further comprising: a first optical fiber mounted on the substrate in one of the grooves optically substantially coaxial with the first GRIN segment; and a second optical fiber mounted on the substrate in one of the grooves optically substantially coaxial with the second GRIN segment.
 26. The apparatus of claim 25, further comprising: a substantially transparent embedding medium substantially filling an optical path between the distal end face of the first GRIN segment and a proximal end face of the first optical fiber; and a substantially transparent embedding medium substantially filling an optical path between the distal end face of the second GRIN segment and a proximal end face of the second optical fiber.
 27. The apparatus of claim 24, further comprising: a substantially transparent embedding medium substantially filling an optical path between the first end face of the optical component and the proximal end face of the first GRIN segment; and a substantially transparent embedding medium substantially filling an optical path between the second end face of the optical component and the proximal end face of the second GRIN segment.
 28. The apparatus of claim 23, wherein the first and second GRIN segments are optically substantially coaxial, further comprising an optical component mounted on the substrate between the proximal ends of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively, the first and second end faces each being substantially normal to a propagation direction defined by the first and second GRIN segments.
 29. The apparatus of claim 23, wherein the first and second GRIN segments are optically laterally displaced from one another, further comprising an optical component mounted on the substrate between the proximal ends of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively, the first and second end faces each being non-normal with respect to a propagation direction defined by the respective first and second GRIN segments, refraction at the first and second end faces of the optical component defining an optical path through the optical component between the first and second GRIN segments.
 30. The apparatus of claim 23, further comprising: a second substrate having at least one groove, the first substrate and the first and second GRIN segments being mounted on the second substrate with the first and second GRIN segments in at least one groove of the second substrate; an optical component positioned between the proximal end faces of the first and second GRIN segments, the optical component having first and second end faces facing the proximal end faces of the first and second GRIN segments, respectively; a first transmission optical element positioned on the second substrate optically substantially coaxial with the first GRIN segment, the first transmission optical element having a proximal end face facing the distal end face of the first GRIN segment; and a second transmission optical element positioned on the second substrate optically substantially coaxial with the second GRIN segment, the second transmission optical element having a proximal end face facing the distal end face of the second GRIN segment.
 31. The apparatus of claim 30, wherein: the first transmission optical element comprises an optical fiber mounted on the second substrate in one of the grooves thereof and optically substantially coaxial with the first GRIN segment; and the second transmission optical element comprises an optical fiber mounted on the second substrate in one of the grooves thereof and optically substantially coaxial with the second GRIN segment.
 32. The apparatus of claim 30, wherein: the first transmission optical element comprises a first planar optical waveguide formed on the second substrate and optically substantially coaxial with the first GRIN segment; and the second transmission optical element comprises a second planar optical waveguide formed on the second substrate and optically substantially coaxial with the second GRIN segment.
 33. The apparatus of claim 30, wherein: the first transmission optical element comprises a planar optical waveguide formed on the second substrate and optically substantially coaxial with the first GRIN segment; and the second transmission optical element comprises an optical fiber secured to the second substrate in one of the grooves and optically substantially coaxial with the second GRIN segment.
 34. The apparatus of claim 30, further comprising: a substantially transparent embedding medium substantially filling an optical path between the first end face of the optical component and the proximal end face of the first GRIN segment; and a substantially transparent embedding medium substantially filling an optical path between the second end face of the optical component and the proximal end face of the second GRIN segment.
 35. The apparatus of claim 30, further comprising: a substantially transparent embedding medium substantially filling an optical path between the proximal end face of the first transmission optical element and the distal end face of the first GRIN segment; and a substantially transparent embedding medium substantially filling an optical path between the proximal end face of the transmission optical element and the distal end face of the second GRIN segment.
 36. The apparatus of claim 30, wherein the optical component, the first and second GRIN segments, and proximal portions of the first and second transmission optical elements are encapsulated.
 37. The apparatus of claim 30, wherein the optical component is mounted on the first substrate.
 38. The apparatus of claim 30, wherein the optical component is mounted on the second substrate.
 39. The apparatus of claim 30, wherein the optical component comprises an optical isolator.
 40. The apparatus of claim 30, wherein: the optical component comprises a Faraday rotator and a linear polarizer; at least one of the transmission optical elements comprises a polarization-selective planar waveguide formed on the second substrate; and the Faraday rotator, polarizer, and polarization-selective planar waveguide together function as an optical isolator.
 41. The apparatus of claim 30, further comprising a laser source optically coupled to the first transmission optical element at a distal portion thereof so that optical output power from the laser source is transmitted into the second transmission optical element through the first transmission optical element and the optical component.
 42. The apparatus of claim 30, further comprising an optical transceiver optically coupled to the first transmission optical element at a distal portion thereof so that optical power is transmitted between the optical transceiver and the second transmission optical element through the first transmission optical element and the optical component.
 43. The apparatus of claim 30, further comprising a photodetector optically coupled to the first transmission optical element at a distal portion thereof so that optical output power is transmitted from the second transmission optical element to the photodetector through the optical component and the first transmission optical element.
 44. The apparatus of claim 23, wherein the GRIN optical medium is a GRIN optical fiber.
 45. The apparatus of claim 23, wherein the first and second GRIN segments comprise segments of a common GRIN optical medium.
 46. A method for making an optical apparatus, comprising: positioning a first transmission optical element on a substrate; positioning a second transmission optical element on the substrate; mounting an optical component on the substrate between respective proximal ends of the first and second transmission optical elements; mounting at least one focusing optical element on the substrate between the respective proximal ends of the transmission optical elements for transmitting optical power between the first and second transmission optical elements through the optical component.
 47. The method of claim 46, wherein at least one of the first and second transmission optical elements comprises an optical fiber mounted on the substrate with its proximal end in a groove on the substrate.
 48. The method of claim 47, wherein the focusing element comprises a GRIN optical fiber segment spliced onto the optical fiber and forming the proximal end thereof.
 49. The method of claim 48, further comprising substantially filling an optical path between the optical component and the proximal end of the optical fiber with a substantially transparent embedding medium.
 50. The method of claim 46, wherein at least one of the first and second transmission optical elements comprises a planar optical waveguide formed on the substrate.
 51. The method of claim 50, wherein the focusing element comprises a focusing segment of the planar optical waveguide at a proximal end thereof.
 52. The method of claim 50, wherein a proximal segment of the planar optical waveguide comprises an optical mode expander.
 53. The method of claim 52, further comprising substantially filling an optical path between the optical component and the proximal end of the planar optical waveguide with a substantially transparent embedding medium.
 54. The method of claim 46, wherein the focusing element comprises a segment of GRIN optical medium mounted on the substrate in a groove on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 55. The method of claim 54, further comprising mounting a second GRIN segment on the substrate in a groove on the substrate between the optical component and the proximal end of the second transmission optical element and optically substantially coaxial with the second transmission optical element.
 56. The method of claim 54, further comprising, prior to mounting the GRIN segment on the substrate: securing a length of GRIN optical medium onto a GRIN substrate; dividing the GRIN optical medium to form the GRIN segment, the GRIN segment remaining secured to the GRIN substrate after mounting the GRIN segment on the substrate.
 57. The method of claim 54, the GRIN optical medium comprising GRIN optical fiber.
 58. The method of claim 54, further comprising: substantially filling an optical path between the optical component and the GRIN segment with a substantially transparent embedding medium; and substantially filling an optical path between the GRIN segment and the proximal end of the first transmission optical element with a substantially transparent embedding medium.
 59. The method of claim 46, wherein the focusing element comprises a substantially spherical ball lens mounted on the substrate in a pocket formed on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 60. The method of claim 46, wherein the focusing element comprises an aspheric lens mounted on the substrate in a pocket formed on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 61. The method of claim 46, wherein the focusing element comprises a Fresnel lens mounted on the substrate in a groove on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 62. The method of claim 46, wherein the first and second transmission optical elements are optically substantially coaxial, the optical component being mounted with its end faces substantially normal to a propagation direction defined by the first and second transmission optical elements.
 63. The method of claim 46, wherein the first and second transmission optical elements are optically laterally displaced from one another, the optical component being mounted with its end faces non-normal with respect to a propagation direction defined by the first and second transmission optical elements, refraction at the end faces of the optical component defining an optical path through the optical component between the first and second transmission optical elements.
 64. The method of claim 46, further comprising mounting a second focusing optical element on the substrate between the respective proximal ends of the transmission optical elements so that the optical component is between the focusing optical elements, the second focusing optical element transmitting optical power between the first and second transmission optical elements through the first focusing optical element and the optical component.
 65. The method of claim 46, wherein the optical component comprises an optical isolator.
 66. The method of claim 46, wherein: the optical component comprises a Faraday rotator and a linear polarizer; at least one of the transmission optical elements comprises a polarization-selective planar waveguide formed on the substrate; and the Faraday rotator, polarizer, and polarization-selective planar waveguide together function as an optical isolator.
 67. The method of claim 46, further comprising optically coupling a laser source to the first transmission optical element at a distal portion thereof so that optical output power from the laser source is transmitted into the second transmission optical element through the first transmission optical element and the optical component.
 68. The method of claim 46, further comprising optically coupling an optical transceiver to the first transmission optical element at a distal portion thereof so that optical power is transmitted between the optical transceiver and the second transmission optical element through the first transmission optical element and the optical component.
 69. The method of claim 46, further comprising optically coupling a photodetector to the first transmission optical element at a distal portion thereof so that optical output power is transmitted from the second transmission optical element to the photodetector through the optical component and the first transmission optical element.
 70. The method of claim 46, further comprising encapsulating the optical component, the focusing element, and proximal portions of the first and second transmission optical elements.
 71. An optical apparatus, comprising: a substrate; a first transmission optical element positioned on the substrate; a second transmission optical element positioned on the substrate; an optical component mounted on the substrate between respective proximal ends of the first and second transmission optical elements; at least one focusing optical element mounted on the substrate between the respective proximal ends of the transmission optical elements for transmitting optical power between the first and second transmission optical elements through the optical component.
 72. The apparatus of claim 71, wherein at least one of the first and second transmission optical elements comprises an optical fiber mounted on the substrate with its proximal end in a groove on the substrate.
 73. The apparatus of claim 72, wherein the focusing element comprises a GRIN optical fiber segment spliced onto the optical fiber and forming the proximal end thereof.
 74. The apparatus of claim 73, further comprising a substantially transparent embedding medium substantially filling an optical path between the optical component and the proximal end of the optical fiber.
 75. The apparatus of claim 71, wherein at least one of the first and second transmission optical elements comprises a planar optical waveguide formed on the substrate.
 76. The apparatus of claim 75, wherein the focusing element comprises a focusing segment of the planar optical waveguide at a proximal end thereof.
 77. The apparatus of claim 75, wherein a proximal segment of the planar optical waveguide comprises an optical mode expander.
 78. The apparatus of claim 77, further comprising a substantially transparent embedding medium substantially filling an optical path between the optical component and the proximal end of the planar optical waveguide.
 79. The apparatus of claim 71, wherein the focusing element comprises a segment of GRIN optical medium mounted on the substrate in a groove on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 80. The apparatus of claim 79, further comprising a second GRIN segment mounted on the substrate in a groove on the substrate between the optical component and the proximal end of the second transmission optical element and optically substantially coaxial with the second transmission optical element.
 81. The apparatus of claim 79, wherein the GRIN segment is formed by securing a length of GRIN optical medium onto a GRIN substrate and then dividing the GRIN optical medium to form the GRIN segment, the GRIN segment remaining secured to the GRIN substrate after mounting the GRIN segment on the substrate.
 82. The apparatus of claim 79, the GRIN optical medium comprising GRIN optical fiber.
 83. The apparatus of claim 79, further comprising: a substantially transparent embedding medium substantially filling an optical path between the optical component and the GRIN segment; and a substantially transparent embedding medium substantially filling an optical path between the GRIN segment and the proximal end of the first transmission optical element.
 84. The apparatus of claim 71, wherein the focusing element comprises a substantially spherical ball lens mounted on the substrate in a pocket formed on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 85. The apparatus of claim 71, wherein the focusing element comprises an aspheric lens mounted on the substrate in a pocket formed on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 86. The apparatus of claim 71, wherein the focusing element comprises a Fresnel lens mounted on the substrate in a groove on the substrate between the optical component and the proximal end of the first transmission optical element and optically substantially coaxial with the first transmission optical element.
 87. The apparatus of claim 71, wherein the first and second transmission optical elements are optically substantially coaxial, the optical component being mounted with its end faces substantially normal to a propagation direction defined by the first and second transmission optical elements.
 88. The apparatus of claim 71, wherein the first and second transmission optical elements are optically laterally displaced from one another, the optical component being mounted with its end faces non-normal with respect to a propagation direction defined by the first and second transmission optical elements, refraction at the end faces of the optical component defining an optical path through the optical component between the first and second transmission optical elements.
 89. The apparatus of claim 71, further comprising a second focusing optical element mounted on the substrate between the respective proximal ends of the transmission optical elements so that the optical component is between the focusing optical elements, the second focusing optical element transmitting optical power between the first and second transmission optical elements through the first focusing optical element and the optical component.
 90. The apparatus of claim 71, wherein the optical component comprises an optical isolator.
 91. The apparatus of claim 71, wherein: the optical component comprises a Faraday rotator and a linear polarizer; at least one of the transmission optical elements comprises a polarization-selective planar waveguide formed on the substrate; and the Faraday rotator, polarizer, and polarization-selective planar waveguide together function as an optical isolator.
 92. The apparatus of claim 71, further comprising a laser source optically coupled to the first transmission optical element at a distal portion thereof so that optical output power from the laser source is transmitted into the second transmission optical element through the first transmission optical element and the optical component.
 93. The apparatus of claim 71, further comprising an optical transceiver optically coupled to the first transmission optical element at a distal portion thereof so that optical power is transmitted between the optical transceiver and the second transmission optical element through the first transmission optical element and the optical component.
 94. The apparatus of claim 71, further comprising a photodetector optically coupled to the first transmission optical element at a distal portion thereof so that optical output power is transmitted from the second transmission optical element to the photodetector through the optical component and the first transmission optical element.
 95. The apparatus of claim 71, wherein the optical component, the focusing element, and proximal portions of the first and second transmission optical elements are encapsulated. 